This goes all the way back to the early days of the registry in win98. The way they described fragmentation was like this:
Picture building a book shelf. You build it only when you add new books to it. So lets say you have a bookshelf that has 4 levels and each holds 20 books. Now lets say that you decide to remove 10 books aka programs. They leave their footprint behind in the form of dust and if in the sun, the suntan line on the bookshelf (these are the leftovers from programs that don't truly uninstall every single thing from the registry - after all a company isn't going to spend many days and work hours working on a perfected uninstall program, after all that isn't what they want you to do with their product, also patches/updates/hotfixes will always cause new stuff to be left behind). Anyhow. Now you have instead of 4 levels with 20 books, you have 3 levels with 20 books and another level with 10 books (20-10). Now since you obviously won't want to cut the space those 10 books created from your bookshelf, so you leave the space in there. There is your fragmentation. Now when you scan the shelf for the book you want, you still will initially scan the first 10 books and will for a second or two start to scan the empty space only to realize you have nothing there, hence the slight delay in the performance due to fragmentation. Now the next 10 books you buy just won't fit in the space that is open (only way to explain why registry entires don't overwrite and fill in the space left behind by old/deleted registry keys - unlike a hard drive which will overwrite unused or vacated space).
So you build a 5th level. However you still have that gap of 10 books on one of the levels. Since those 10 new books had to go to another shelf you have now increased the scanning time of your book titles.
So the gap on the shelf is your fragmentation and loss of performance. You might see a reduction in the actual size of the registry, MB wise. However unless every single entry from that program is removed you have keys being loaded that don't need to be, hence a loss in performance. Now supposedly Win 7 was suppose to do a better job of compacting itself to fill in these emtpy spaces.` However Win 7 can only handle the most basic of entries and only those that actually are ones that they are aware of. For example they don't know all the entries Norton makes, Mcafee, Paintshop Pro, and so on. Sure they could try to run one of dozens of supposed "registry change logging" programs but none are perfect, trust me I have tried nearly all that were out there and none were ever the same in catching.
Also don't forget that programs leave behind in the registry (depending on the program of course) any former serial numbers, activations, etc. Since I have NDA with several companies that use this I can't provide company or programs that this happens with. In some cases it is simply used to help you reinstall later, however in other cases it is used as an effective anti-piracy method, as you will not be able to find exactly where the serial number that has been deemed invalid is being stored. So you can't get rid of it and put a new one its place or do other tricks once the program has invalidated that serial number.
So there will always be space left behind in the reg, now multiply that space by 2 years of uninstalling and reinstalling applications, drivers, etc. and you can see the mess that is left behind and created. So everytime someone uninstalls NIS and reinstalls it they are creating minute gaps, which that moment do not have any impact on performance. But since it will certainly not be the only program you install, uninstall and reinstall in your OS lifetime, it will eventually add up.
Now if this explaination has changed since Windows 7 Sp1 has been released then I need to update my personal KB on the registry. Since registry "fixing" programs are totally off base and inaccurate. If you don't believe this (not you huwyngr) do a fresh clean install of windows xp, vista or 7 (32or64) then install the registry scanning/fixing/error detecting program. Run it and it will find dozens of "errors" "broken links" "orphaned" or whatever wording they use. Now unless you are intimately familiar with the entire Windows Registry (to be honest I don't think even Bill Gates could tear down a registry and tell you exactly what is or isn't needed) you have to investigate each and every key they named as having a problem. There IS NO guarantee that deleting that key will make any difference. That is one reason the reg scanning/fixing programs have never really provided clear guidance on which entries can truly be erased. The company knows that it too great a liability that they get you only 1/2 to the solution and then suggest you back up the registry/computer before you make any changes and why they make sure you know in the EULA and in other dialogs of the program that they aren't responsible.
Anyhow, back to the real issue, you would think that Symantec/Norton would take a look at the 2011 program where the program control works properly, then do a comparison of code that could affect it in 2012 to zero in and eliminate the problem. Since they are holding the code in their hands and on their computers, they can instantly test a change in code to see first how it affects NIS and so on. It shows that they didn't really test their program out to the fullest extent or that because they are running "isolated" pre-configured computers they are not seeing the program in the real world and hence it is not a surprise they were not able to recreate the issue. They should have been all over the problem the moment the first person reported it. They should have done a remote session and see the error. Then issue the NRnR tool, make sure no other companies security software is on it and then have them recreate for the tech. If that person can recreate it all the time, then if another person does as well they should already be building a huge database of info on the problem. Since they understand better than probably 99% of us what is actually going on in the code or in the background when you push on the Program Control button.
So what is a person suppose to do, uninstall 2012, install 2011 for now, then maybe if we are lucky, in August they fix the problem, so you uninstall 2011 and reinstall 2012, then in Sept 2013 comes out and offers better protection than 2012, so you uninstall 2012 and install 2013. You are almost certain to leave fragments from each version in your registry and one of them could actually come back to haunt you in the form of an error/crash/bsod that references some other program and that the other company can't seem to find what is causing the problem. Plus with Norton making no export import key for program control rules, all those entries have to be re-entered in 2011, 2012 and 2013 and that is assuming each install works out perfect.
