You can be amazed what you can make out of an old second hand computer. I once found a desktop in a dumpster that was full of sawdust, with no side cover. I blew all the dust out, and found out that the only thing that was wrong was the power supply. Note that when getting older computers, it may be harder to find parts for. A good thing to do is learn from frame up. like what motherboard fits this case, what cpu fits this motherboard, what graphic card fits this motherboard, memory, etc. Knowing how to build a computer simply by understanding what fits in it, is key to saving time and money. You will make less mistakes when purchasing. But back to the topic of a cheap desktop..well i have seen newegg sell standard dual core intel celeron PCs that you always see offices, and POS registers. I have seen them as low as 89 dollars. They are standard as it gets, but its a good place to start.
