You may wish to setup your system so it's virus improbable.
One thing you could do is a fresh install of xp, I suggest windows black, makes light, really light work out of xp install. Or if you'd like to continue using your system, more power to you.
Clear your startup, defrag, create restore points, don't trollolol the intrawebz on your admin account. --Create a regular user and use that from now on. Do regular updates, virii scans, ect.
If you'd like to continue using malware bytes, right on, if not I suggest blink.
Run nessus or retina scans on your system, harden it. Perhaps try and "packeted" yourself your machine to make sure your firewall is doing its job.
Keep A live copy of linux/FreeBSD handy incase anything gets out of control. Another handy disk to have is UBCD (Ultimate Boot CD), has alot of useful programs like gag, for mbr restores, testdisk for partition recovery, etc, etc. (not all problems give you the luxury of being able to boot into the system.)
Alot of virii manipulate existing anti-virus exploits, click the injector link in my sig that says "inj3ct0r" and do some searches of the antivirus programs you know/have heard about.
You will be presented with a list of all the bugs found in each, you decide, which one you think has the least ammount of exploits/safest/best av.
Cheers, 1ce