Hi,
I found out recently a solution for the two WAST issues on XP: there are problems both with counters and clients features. Like on XP, Vista's issues are related with the old Windows 2000 interface to a library and with the security DCOM settings. The solution for XP problem is probably similar for Vista/W2003/W2008:
1. You must copy pdh.dll in the WAST directory to use performance counters
2. You must set appropriate DCOM permissions to use remote clients
I described all on my blog:
http://www.maurodalfreddo.it/archives/249/web-application-stress-tool-anche-su-xp
I'm sorry, it is only in Italian, but it's very simply to read and understand; I will translate some article in the future
I hope this help you