Microsoft will end Windows XP support by April 2014. Your old hardware is probably not powerful enough for Windows 8.1. In this case you can still upgrade software with latest version of GNU/Linux. Best of all is is free software. There is no cost to use GNU/Linux.
There are many kinds of GNU/Linux operating systems. Check out DistroWatch for a comprehensive list. Each distribution was created fo a specific end user requirement. Look around and compare to get the best GNU/Linux distribution that suites your needs. Please contact us when there are questions or more information is needed.