Right think I've cracked the unable-to-uninstall-IE7Beta2 issue.
I deleted the IESetup and Internet Explorer regkeys from HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft. Then enabled View Hidden Files and Folders (in Tools | Folder Options | View). Renamed iexplore.exe in C:\Program Files\Internet Explorer, and rebooted.
Seems to trick IE7B3 into believing BETA 2 has been unistalled. Worked for me anyhow.
Your mileage may vary!