The Uninstallation section in the readme is a start:
http://download.microsoft.com/download/B/5/3/B5379D88-51D7-4240-9423-CE1427502784/VS2010Beta2Readme.htm#UninstallingHere's the things I would try - though backup first. Any mucking around with program files, registry, and windows folders may well totally hose the machine and require an OS reinstall anyway.
Make sure the 'c:\program files\Microsoft Visual Studio 10.0' folder is deleted.
In regedit, under the 'HKEY_CURRENT_USER\Software\Microsoft\VisualStudio' key, delete all 10.0 folders
The 4.0 CLR runtime folder is 'c:\Windows\Microsoft.NET\Framework\v4.0.20506' but I don't think you'll be able to get rid of that due to system file protection. Maybe in a safe-boot though you may end up really hosing something and need to reinstall the OS.
If all that doesn't work, you're probably going to have to reinstall the OS. I imagine you had some sort of install/uninstall failure that's left you in an undeterminate state so that VS won't run properly and it won't uninstall properly. Unfortunately that's a risk of beta software.