One thing I have discovered is that if you use Apple/Adobe/Microsoft on the Mac side then you may need to check for updates using other Mac accounts. I had an Adobe update that did not want to install till I used a different user account. I had a MS Office 2004 update that did not present itself till I logged in as a differnt user, even though other machines had it.
What I would do is;
1 - Uninstall Parallels. They have a list of files to check are also removed.
2 - Restart with safe boot. Check the Parallels files are gone. (Not you VM).
3 - Restart normally and login as a user with administrative rights.
4 - Run Software Update and install. May need a number of restarts and reruns till you are told no more updates.
5 - Run Adobe updater and install updates.
6 - If you have MS Office for Mac 2004 (or their new one) run MS Autoupdater.
7 - Restart.
8 - If you have more than one account with administrative rights, login with a different account and try steps 4 to 7 using that account.
9 - Install Parallels using latest.
10 - Start Parallels and login to XP with Administrator (Just to make sure the ParallelTools install OK). Restart XP when asked to.
11 - Shutdown XP and close Parallels.
I would say that this should point the finger somewhere if it works, or where you may look if it doesn't.
If you had Apple updates for Quicktime, Safari or iTunes, then Parallels has a problem with one of the Kernel drivers installed in an earlier Quicktime/Safari/iTunes update.
If you get an MS Office update, especially with using a second mac user to check, then the MS audaemon does something that Parallels does not like and is a MS sourced problem.
If you get an Adobe update, it may fix a memory allocation issue that could cause interferance with Parallels.
I had an issues that needed multiple accounts and softwrae installs to rectify. "Parallels crash causes 20 min boot"
http://forums.parallels.com/showthread.php?t=20503
Good luck