NOTE: This post releates to Parallels 3
Often when I resume a parallels session (always when I move from one wireless network to another, I think) the networking breaks in my Windows XP VM.
For some reason pvsnatd on my Mac Book Pro seems to freak out, restarting the process always fixes the problem.
To restart pvsnatd, open up Terminal and enter the command below.
sudo /Library/StartupItems/Parallels/pvsnatd restart
You should now be back in buisness :)