Buying a used computer with XP is probably your best direction to go. Any new computer build may lack the drivers to support XP as the chips that the drivers drive may only support Windows 7 and newer as an example.
So look for a good safe buyer to buy a used Windows XP OS computer from and run the games you want to play on XP on that system. I would suggest something like a Quadcore CPU Like the Intel Q6600 and a NVidia GTX Video Card that pairs up well with your games. if the games are not extremely heavy in graphics you might be able to get a GT card like a 9800GT videocard. Look at the games you have for suggested hardware and have hardware equal to or better than the suggested hardware requirements.
I would be sure that the computer that you get has SATA Controller so that your not buying a IDE aka PATA drive system which is lesser performance as well as SATA drives come in larger than 500GB capacity whereas IDE aka PATA drives come in 500GB or less in size and slower.
Other thing to know is that some older games require some system changes to run correctly such as I like to play Unreal Tournament 99 and this game was created back when most computers were a single core processor. When running this game on Windows XP on a Athlon II x4 620 Quadcore 2.6Ghz CPU the game would stutter. I had to set core affinity to force the game to run on 1 of 4 cores in order for the game to play correctly.
