Norton possibly corrupting hyperlinks in emails

I wonder if you can help on a problem I have.
 
To do with sending out a news bulletin.
 
I used to just do it as a pdf, and attach it as an email, and give a link to it on the website.  That was nice and easy and caused no problems. All worked fine.
 
However, it seemed better to try and do it in a more user-friendly way, and the way some other electronic newsletters are done, so there is an email, containing the stories with a heading and a bit of text, and a link to the rest of the item.
 
I found a way to do it (not the best way !) by putting the articles onto web pages my website, and doing hyperlinks to them from an email, which is sent out to the mailing list.
 
But the main problem is that the hyperlinks have been scrambled. 
 
They started off fine, and I tested them several times, by emailing myself and also my other email address    All the links worked.
 
Then I sent out the bulletin, (I send it out as dozens of separate emails, each to not more than 15 or so addresses, as I was told that is better than sending it out to a huge number .... not sure if that is correct ?)
 
And then I find that in some of the emails the links work, and in others that appear identical (just a copy of the same email) the links all start with mhtlm, as below:
 
 
mhtml:{61FB8201-9863-42A3-B903-9CE0C3124D2E}mid://00000074/!x-usc:   (followed by the right url)
 
 
 
http://www.airportwatch.org.uk/news/detail.php?art_id=2091  (this is the hyperlink in one of the emails that works)
 
 
and how it looks in the ones that don't work:
 
mhtml:{61FB8201-9863-42A3-B903-9CE0C3124D2E}mid://00000074/!x-usc:http://www.airportwatch.org.uk/news/detail.php?art_id=2091
 
 
 
I have tried to do a bit  of research on the internet about what mhtlm is, and MIME, but I am  afraid it way above my technical competence !  I cannot find anything useful to explain my problem, through any number of google searches.  Not quite sure what question to ask.
 
Also, Norton (which was turned on to scan outgoing emails) blocked at least 10 emails, and I had to re-send them.  They did go the second time of trying.
 
I have Vista Home Premium as my operating system on my computer, and my email system is Windows Mail.
 
When I sent out the emails, the system asked each time if each email should go "Send As Is", or   "Send at Unicode" .    I tried both, and there seemed to be no difference in the links, so I send them as Unicode.  Not sure what difference that might make ?
 
Is my problem caused by Norton corrupting the links?
 
Any ideas or suggestions gratefully received.
 
Best wishes
 
Sarah Clayton