AVG Free is a very good anti-virus that is widely used but it does require SP2 for Windows XP. If downloading it make sure that you use the AVG site click here
Some forum members have found that the latest version (8.5) slows down there PCs and/or have had update issues and so have recommended alternatives. Search on the forums for AVG and you will find the many posts with their suggestions. I use AVG Internet Security (paid for version) on Windows XP with SP2 and have never had any problems, though you have to be a bit patient on start-up, but that's true of most anti-virus programs. I have also installed AVG Free on Windows 7 without problem.