Don't be too hasty to reinstall your OS, I'm running XP and last time I cleaned it up I had 800mb free space (On a 80Gb hdd aint that much) and about 32 services running, I've cleaned up the hard drive and made it to a startup services of 17 things. My machine is running OK giving me enough time to get some new hardware (I'm going to be installing a radeon 9600 pro to replace my already outdated FX 5200) in this way I limit the probability of driver issues when I upgrade plus backing up 46 Gb of stuff isn't always that easy... so untill I get my new hard drive I'll be living on borrowed time (Considering the amount of malware I've just disabled and incorerectly deleted.) but it works for me NOW and I dont have to do an inconvienient reformat when I'm unprepared.

Ultimately I'd reinstall My OS about once every 4 - 8 months simply becuase it's easier than maintaining some kind of realistic file structure and cuase I'm to lazy to fix the problems properly