My old Win95, Win98, Win2K machines always used to slow down over time to the point that it was well worth backing up documents, photos etc. then reformatting the hard drive and reinstalling windows.
I always rebuilt such machines every 12-24 months as I found that it gave them a new lease of life - and despite being very careful on viruses, malware etc. there seemed no way to avoid a general "slowing down" over time.
Must say that my WinXP box doesn't appear to suffer from this at all - my current installation is now now 24+ months old, and as Gertrude says so long as you keep it clean it should just keep on going
RC