Get Spybot Search & Destroy from
http://www.safer-networking.org/ . After installing, go to simple mode and get any updates the program needs and do a scan of your pc for spyware. Also check out
http://www.sysinfo.org/startuplist.php, for a list of files that start up with your system to determine what you need and don't need.
I would check out Black Viper's excellent services guide to eek out even more performance from your machine. The guide is available at
http://www.blackviper.com/
Oh ya, check for viruses and trojans too (spybot won't find either). I've found that TrendMicro's Houscall was a life saver when fixing other ppl's systems.
http://housecall.trendmicro.com/