Three strong recommendations, and a couple of maintenance tips.
1. Get Diskeeper ($20) and defrag everyday before you shut down. With regular use, this should only take a couple of minutes. The native defrag is Diskeeper Lite; it is not as aggressive as the full version.
2. Get Spy Sweeper ($30) and use it everyday. Get rid of the freebies. I'll bet this one will find lots of stuff AdAware & SpyBot miss.
3. Get Registry Fix($$) or Registry Mechanic($$) and run it every other day.
At least every three days go to Start - Run- (type in) %temp% and delete EVERYTHING in there it will allow.
At least every three days go to Windows/Prefetch and Windows/Temp and delete everything in there it will allow.
Run defrag after those operations.
Follow site3op's advice. Go into Add/Remove Programs and take out anything you don't use. Check Device Manager (right-click My Computer - Properties - Hardware) for problems. Disable stuff you don't use.
Another thing I do with my customers is a "Disaster Recovery" back-up every six months. NOT AN IMAGE RESTORE IN THIS CASE. (We use images as regular backups almost every day). I use Back Up My PC v6 (there are others). Important to note; you must use the DR option. Much faster and less work to do; restoration to exactly what you were. You'll need a floppy drive, four floppies, and an XP install disc with key. Be sure to delete the partition first, recreate it, then use long format. I find it refreshes the hd and tightens things up. Start to finish - 2 hours.
You've got to be pro-active in taking care of your comp every day. You can't just turn it on each day.