I had the same problem yesterday with a t-com hotspot. The following solution worked for me.
Use host only networking with dhcp on windows xp.
After that configure internet sharing of airport with en2 (this is the Parallels Host-Guest Adapter).
In network configuration I have Parallels Host-Guest Adapter listed before airport.
You can try this. (But I do not know if this matters.)
After that XP virtually had the same ip as mac os and everything worked fine.
Last edited: Jun 22, 2006