FD, did you start with all the simple stuff like:
Start > Programs > Accessories > System tools > Disk clean?
-or-
Start > Run > type "msconfig" then enter. Go to "startup", disable everything you don't need, reboot, see how she runs.
I've had a system or three so buggered up I had to reboot into safe mode first, then do the above.
FWIW, I always use FireFox for my browser, AVAST for my anti-virus, windows firewall and I run SpyBot from time to time to eliminate crap from my system.
I'm certainly no PC expert, but I rarely have problems when using everything above.