Turn off Aero. It might help (did for me, when I still had 512 MB RAM

. Now, with 2.5 GB RAM, Aero + aRO works fine.).
Well, there is a problem for some WLAN adapters: Whenever connected to the internet, the sound and screen lags every 20-30sec. But only with WLAN, when I'm connected via cable, everything works fine.
(Today I "fixed" this problem somehow... though, I need 2 PCs for that now @_@)
Make sure nobody else is using the internet. Oh, and do not download/upload things while playing.