Hi there! Definitely try getting rid of any SpyWare first ... it slows down my computer significantly.
I use AdAware (the free version) and it's a lifesaver. I run it once a week and it eliminates between 50 and 200 spyware infections each time.
You can read about it and download it here:
Ad-Aware 2008 - Free software downloads and reviews - CNET Download.com
It gets pretty good reviews.
Good luck!