I have already tried to mine with WinXP(SP3), but I had some problems that I don't know if they were dependent from software or hardware. I mined very well with a HD5870 (about 470Mhash/sec) and GUIminer (OpenCL miner) (OC 1050MHz... this card is equipped with Accelero XTREME ---> 52°C at 75% fan speed :-) very good!).
Now I've problem with my HD5970. Moreover, seems that only one of two GPUs on this card, works with this configuration... every time that I change videocard on my system, I need to reinstall Catalyst & SDK... what a mess!
Configuration Info for HD 5870: Catalyst 10.7 + SDK 2.1
Configuration Info for HD 5970: Catalyst 11.2 + SDK 2.2
Always with GUIMiner & OpenCL miner
From SDK version 2.3, WinXP is not longer supported...