Potential Problems with Used Computers
When considering buying a used computer being offered to you by a friend or family member for a really low price, you'll first want to determine why they're selling it. If the reason is that they've upgraded and have no use for it, then that is a safe one. If their reason, however, is that they are having problems with it that they are "sure you can figure out", be wary. Even if you are a computer expert, don't commit to buying a computer that could have problems that will cost you money to fix. If possible, see if you can fix the problem before forking over your cash.
A common reason the less technical savvy out there dump their computers is because their operating system became corrupted, either through a malfunction or virus. The less "in the know" think that if they do not have the original operating system disk, then there is no way to recover the computer without buying a completely new OS. Of course, this isn't the case, and any OS disk of the same type (ie: XP Pro 64 bit, Vista Home Basic, etc) will work for a fresh install, so long as you have the serial key (which can often be found printed on a sticker on the back of the machine).
If the computer being offered to you was recently considered a good or "new" one, but won't power on, you can probably pick it up and repair it for very little money. A very frequent hardware problem that can effect computers at any age is a failed power supply. There are great articles here on Bright Hub to help you troubleshoot computer hardware problems, and potentially find a very low cost solution.
If the computer you're considering is in good working condition, then the only thing that you should try to make sure and get along with it is the documentation, operating system, and driver disks, if at all possible. This will make anything you need to do with the computer in the future much, much easier.