We have a nasty bug which prevents customers from Installing Visual Studio 2005 on a Windows XP 64 bit machine.
This has been fixed recently but didn't make it in for our June CTP.
If you're trying to install on an XP 64 bit machine, the work around is:
To workaround this you can change registry at HKLM,System\CurrentControlSet\Control\Windows value CSDVersion to 512. install VS, then change it back.
I need to give Andrey (one of the devs on our setup team) props for figuring out this workaround :)