Parallels 4 in Snow Leopard

I have an iMac on which I had Parallels 3 but I made the mistake of upgrading to Snow Leopard. I couldn't access my Parallels 3 VM, so I was able to get Parallels 4 and load it. It seems to have converted my Parallels 3 files, but when I go to open it, I get the following error message "Unable to load Parallels driver. Please restart your computer." I've restarted multiple times and still get the same error message. After that error message, it will give me another that says "Unable to start your virtual machine. Please contact Parallels support."

Any ideas what is going wrong and how to fix it?