It has occurred to me recently that a large number of people I know have been struck by the Aim viruses...and yes...that includes myself (even though it's embarrassing to admit). I've noticed, of those who have unknowlingly contracted this technological microbe and have consequently had it sent to me, that it seems to have appeared in one form -- a link that leads to some kind of picture, but the file name does not end in .jpg or .png or any other image file name. For example, here is a link to a wallpaper I made:
http://img.photobucket.com/albums/v300/lavawingzero/windowswp.jpg
Go ahead, click on it...it's safe
Most of the links that i've seen end with .exe or even one had .bat...but they had the .jpeg before it, so they kinda looked like this
www.somethinganother.com/yeahthisisanexample.jpeg.exe
Once you click it, it asks you to download a program which you can then find where you downloaded and click on to run...and all heck breaks loose on your buddy list.
I can't say for sure whether or not this will be true in every case, but it seems to be the most common situation that I've encountered. Your best bet is to ask the person who sent it to you whether or not they meant to send it before clicking on it. Better yet, just don't click on it and ask them later.
If you do happen to catch hold of a virus, there is a free program called Aimfix -- it's found at www.jayloden.com -- that you can download and run (while in safe mode works best) to get rid of the virus.
I really hope this helps, and if you know anything more about these nasty viruses, feel free to comment!
--dave