It sounds to me as if it's clogged up. By that I mean there's bits of this program here, and bits of that program there. They never completely disappear when you delete them, and there are temp files and a registry file that just grows and grows like a cancer that is really difficult to clean.
My advice is to sit down and backup all your essential data.
Email Address Book
You get the picture.
Save them to CD if you can.
Then; Make sure you have the latest drivers for everything (Motherboard, CD, DVD, Modem etc. etc) on another disk.
Then Double check everything.
Turn off the PC and go to do something else for at least 6 hours.
Come back and check everything again.
Then re-install windows making sure you format your C: drive. Re-install drivers, re-install onlyu the programs you really need/use, re-install your back-ups.
You will forget soemthing, everyone does, but hopefully nothing too important.
You will find that your PC is suddenly 25% faster, but will feel 100% faster (until you clog it up again :))