Result of maleware or infection?

NAV2008 Windows Vista Home Premium OS with Vitsa Service Pack 1

 

There has been a chneg in osme function of my machine and I'm wondering if it could be a sign of maleware or infection?

 

I log on my machine through a limited user account. In the past if I needed to do an update for say Java or an item like real player or even download trial games, I would be prompted by my system for the admin password.

 

 Now I get either no prompt or if I get a prompt and put in my admin password, the function to update or install doesn't go further. 

 

I can still do things like like run a live update for Norton or Windows. I can still open the task manager and show process for all users by putting in my admin password. I can even delete some programs after putting in the admin password.

 

So there are just some functions that I now have to log on as admin such as getting the recent Java update

 

So that brings me to my concern. Is this due to some malware/virus that would want me logged on as admin so that it could do some damage?  I run full scans daily with NAV2008 and they show nothing more than tracking cookies

 

All feedback would be helpful. Thanks