I'm seeing the same symptoms that are described in issue 1543 with the 2.6b3 MSIs. Namely, when you run one of the MSIs (either 32-bit or 64-bit) then the other will refuse to install. This is on XP Pro x64 SP2. python-3.0b3.msi and python-3.0b3.amd64.msi exhibit the same problem. Having 32-bit and 64-bit version coexist makes it much easier to build extensions for and test on both versions at the same time.
Thanks for the reminder. This is now fixed in r66439 and r66440, as well as 2.6rc1 (although I haven't actually tested it, due to lack of access to an AMD64 system right now - please report whether it works).