Groundgripper,
Jotti's website is a godsend.
Every AV program has its strengths and weaknesses, some catch virusses which others dont, however having two AV on your system can cause problems, even cancelling each other out.
You will often find virusses in threes if your system seems to be badly affected. Generally a security loophole will be used to get a backdoor virus on to your computer. The backdoor then opens the way for a downloader virus which can bring some pretty nasty stuff onto your computer. Obviously just because your AV has picked up a downloader it dosn't mean that it has also picked up the backdoor which allowed it on there in the first place....
Sometimes you just have to wait until the AV bods get an update out which identifies the virus you have, though Jotti's ensures that if any of them have a handle on said beastie then you can get rid of it.
Even though all Microsoft products are ****e I suggest you go to their website with IE and pick up any security patches or updates which are available.
Honestly running a Win32 system which is used by multiple users is probably good training for being a professional sysadmin nowadays.