another \decent place is
savemybutt.com

there is a really good program called advance windows care, its good easy to use and its free

cc cleaner is another good cleaning tool

I am using Aviara virus for free and so far ok

and they have some free registry cleaners too

and filehippo.com has everything you could think of, a lot of them are free

one thing I might mention if you are getting a new PC and it has vista, try to get at least 2gigs of memory, if it only has one, time to haggle, just say I would buy this right now if it had 2 gigs, see if they wont add one for you, doesnt hurt to try