Nothing like a clean sweep to freshen up the old PC. I'm building a new one at the moment and moving up to a duo-core processor and the latest socket motherboard.

I'd recommend that after formating your HD you divide it up into a Drive C and to a Drive D
Make Drive:C about 15 Gig and put your O/S only on it. Use Drive D for data, photo's and everything else. Make a folder in D for 'programs' and any programs you download form the web. or from Disc, keep them in this folder.

Next time you need to do a clean install you just scrub and re-format Drive C leaving Drive D with all your Exe. programs ready to load into your O/S from your program folder and all your Data etc. right there on Drive D too [I burn an image using Nero of all the programs onto a disc too in case I ever stuff the HD].

Viruses/Trojans etc. infect the O/S [on C] not your benign data etc. on D [scan it all anyway].

It takes me about 30 min. to set up my PC after a clean install [every 12mths.] using the above method because all I have to load onto the HD is the O/S into Drive C and then load my programs to the O/S from my 'program' folder!

Another worthwhile thing to do is to take an image [back-up] of your registry straight after installing your O/S using ERUNT [free] and do this say once a month. Keep these registry back-up in a folder on Drive D. With ERUNT. You just double click on any of these back-ups and your registry and O/S is back to problem free running. Download ERUNT to disc before you scrub your O/S.