Companies like dell and gateway lease systems then resell these systems cheap after they come back.
Check out the dell outlet link from this home computing page. They hardly ever post the link directly from their frontpage.
http://www.dell.com/content/default.aspx?c=us&cs=19&l=en&s=dhs
Here's the link for gateway's remanufactured and off lease desktops.
http://www.gateway.com/reman/hm_desktop_matrix.shtml

Tigerdirect buys off lease systems from compaq, emachines, acer, etc. and sells complete systems. They set up their pages by popularity so you can find a $2000 system right next to a $200 one. The first page has a compaq system off lease for $249. Check out the barebones deals as well. I've seen motherboards with processor for $70.  Caviat with this place is that many components have rebates. You have to pay $120 for that $70 barebone kit and wait a few months for the check to come back. Sometimes you have to fill out the rebate form in green ink at midnight, under a full moon, while facing west or you haven't filled it out properly and don't get the rebate.
http://www.tigerdirect.com/applications/Category/category_tlcNEW.asp?CatId=6

I recently went looking for a new power supply because I run 3 hard drives and two CDs. I wound up getting the case with 500Watt supply for $50 at BestBuy. The 400Watt power supplies by themself were $70. It was more work to swap everything, but the case was better and had two front ports for USB.

Deals are all around, but you usually have to buy from 2 or 3 supplier to get the best prices. Deals also change constantly so decide how much you want to spend and be ready to go.