Well, if you dont have good specs for XP,(384 mb of pc133 sdram will go just fine though), whatever you do, DO NOT take windows ME (the horror!) because it really sucks.
Well I loved ME until I got XP. I did not like 98 because I had so many problems with it. Now to the topic...I have Windows XP Pro SP2 installed on my sister's PII 350Mhz MMX processor with 128 MB SD-RAM (100Mhz). It worked really good but has slowed down considerably when loading up due to the greater amount of running programs such as messengers, firewall and antivirus. Otherwise it should work just fine.