I would download the free versions of avira and malawarebytes and clean the crap out of the machine. When finished finding all the stuff those find, you might run Microsoft Essentials which is also free to see if it finds anything.
Not sure which operating system you are running, but both win 7 and Ubuntu 11.04 (which is free) run much faster than any of the old stuff.
Ubuntu being linuxed based avoids a lot of the crap associated with windows.
I use it for 99% of my stuff.
If the machine does not have a minimum of 2gs of memory, that will help...I usually put at least 4gs in because memory is cheap these days