I keep seeing people recomend reformatting your hard drive & reloading everything ... pretty stupid if you ask me!
If you have a decent: firewall, anti-virus/ad/spyware, and conservitive clean-up toolset (ie: the free versions of ZoneAlarm/Vista Defender, Avast/AVG, Spybot S+D, AdAware, & CCleaner) and maintain them (ie: download their updates weekly) there should be NO reason to reformat your hard drive.
Of course, you must use caution as to which websites you visit and use cleaners, etc., conservatively ... ie: never ever delete (or let a toolset) delete registry settings, files in the windows directory, etc., unless you are very sure what is being deleted ... or you are asking for problems.
If you visit porn sites, click on/open every spam e-mail, use one user-id and/or password for all the sites you have to sign into, etc., then you are asking for trouble and may get infected anyway; but if that is the case you probably are falling for phishing attacks, etc., are have exposed your SSN, bank acount pin, etc., to identity theft anyway ... in which case problems with your pc should be the least of your concerns.
A once a month cleaning of temp files, etc., and an occasional defrag are all you generally should have to do other than letting the toolsets (firewall, anti-virus/ad/spy software) do their thing.
I can not recommend reformatting/rebuilding unless a disaster has already occured!!! I will strongly suggest creating back-ups of all your important files on a regular basis (just in case the hard drive fails which is probably the most likely major problem you will have if you have/use decent protective toolset).