I agree that the "wipe and reinstall" option is the best way, but I've got all kinds of stuff on here that I don't even remember the source of. Trying to find it all again is a nightmare - I had to do it once, when the laptop's hard drive died, and I had to do it earlier this year when I bought a new home PC. But I'd rather not...
If the machine is taking 20 minutes to boot up, and the HD light isn't flashing, then something pretty basic is wrong. You could switch to Linux (I like Fedora, but I'm not brave enough yet to use it as the primary system - give me time!). Work on the assumption that there's something "adrift"...
Do you have more than one computer, and are they networked? Do you sometimes boot the one with the other turned off, and sometimes with it on?
How full is your hard drive? Double-click "My Computer", click on Drive C, then right-click and select "Properties". If it's over about 75% full, a bigger or a second hard drive is indicated.
How much memory has the machine? XP is 'orrible in less than 512 Meg (but not 20 minutes to boot up 'orrible).
Otherwise, you're into "Hijack this" - see the stickies at the top of this forum.