Once again, Josh has pointed me towards the solution of getting Visual Studio 2005 Beta 1 running side by side with Visual Studio.NET2003.
In this case, I had a clean Windows XP Pro + SP2 RC2 install, installed VS 2003 + all my add-ins, tools, etc., then installed VS 2005 Beta 1.Whenever trying to debug a Web app after the install, I got:
Unable to start debugging on the web server.”
In the debug window, it outputs:
Auto-attach to process ‘ aspnet_wp.exe’ on machine ‘WINXPPRO-VM1’ failed. Error code 0x8013134b.
The solution, found on the forums at asp.net was to simply switch the selected runtime from the new ASP.NET tab in the properties for the default web site in IIS.