I recently tried upgrading Parallels to v3 after installing Leopard and was met with a brick wall every time no matter what I did. Well almost. I would run the installer, click install, enter my password then installation would sit forever at the running installation script stage. That's the very first stage before it actually does anything. I restarted. Ran the uninstaller. Restarted again. Followed the manual uninstall steps. Restarted again. Tried all these things again with another fresh/unused user account. Anyway, I finally cracked into the installation package and opened the preflight script in a text editor since that's where it was getting hung up. It's a shell script that basically just checks to see if various services are running one of which is Internet Sharing. I use Internet sharing to share out my Ethernet port over Airport so I can hook up my iPhone. Once I turned Internet Sharing off the installation went with out a hitch. I don't have the machine specs handy, it's my work machine but can post them up on Monday if needed.