Windows XP is going to eXPire on April 8th 2014. Here are some cheap to free ways to upgrade to a more modern OS. Buying a used PC than a Windows license is in many cases is cheaper.
Second Hand PCs- If you are looking to upgrade from Windows XP cheaply, look for used PCs, specifically Windows Vista and up.
EBay- Is a good place to find a second hand pc. When you are searching, filter the results with either Vista, Windows 7 or Windows 8. Windows Vista PCs will be the cheapest and offer a more modern experience than Windows XP and Vista wont expire until 2017. You can also get Windows 7 pcs for very cheap prices too. If you can find a Windows 7 PC on eBay that's cheap, then go for it. Don't forget to look at sellers feedback before deciding to buy a PC.
Craigslist- Craigslist offers a more local experience for searching for a used PC. Unlike EBay, you can look and test the computer you are interested in before buying. Craigslist can be considerably cheaper than EBay also. Be careful though, Craigslist does not offer any protection for things that you buy, so be sure to test any computer you may purchase. Also, ask for the craigslist seller's phone number to talk to them, this may give better insight to who they are and whether they are a trustworthy seller or not.
Friends/Family- Ask friends or family members to see if they have (Vista and up) computers that they are looking to sell.
Thrift Stores- This may not the best place to find a (Vista and up) PC, but there are pcs that turn up once in a great while.
Tag Sales - Finding a PC that runs Windows Vista and up at a tag sale may be a longshot but it is worth a try.
Local schools upgrading their computers - Local schools sometimes recycle their older computers, so if you can, ask to see if a local school would give you a (Vista and up) computer for free.
Local businesses - Businesses also sometimes recycle their older pcs, ask to see if you could acquire a computer for free. Be sure to check to see if the business's pcs are vista and up.
Friends/Family- Ask around and see if your friends or family members are getting rid of their (Vista and up) PC for free.
Free section of Craigslist - check regularly to see if there are any free (Vista and up) PCs in the free section of craigslist. People who are less knowledgeable about PCs sometimes just give away their suppose broken pc. Sometimes the supposedly broken PC may just be a major software problem or a severe problem with Windows.