Currently the Visual Studio 2008 extensions for Windows SharePoint Services 3.0, v1.2 do not install on an x64 operating system and the download page contains the following note:
Note: This download does not work with an x64 OS. We currently recommend a x86 development machine for SharePoint development when targeting x64 OS for test and production. Output binaries are all .NET assemblies so you can reasonably expect all .NET assemblies built on the x86 dev box to still work on your x86 OS test and production machines. The CLR will JIT compile the .NET assembly on each target machine and will optimize for x64 OS when run on that architecture. Testing on x64 after doing development on x86 is still recommended.
Nevertheless, this remains one of the top requests for SharePoint developers and so we are currently looking at options for addressing it. Currently we would recommend Visual Studio 2008 with the Visual Studio 2008 extensions for Windows SharePoint Services for all SharePoint development and that means using x86 for your development environment. That’s the best option for development now with the best migration path going forward.
Meanwhile if there’s something you need assistance with in the Visual Studio extensions for SharePoint the best place to ask is the SharePoint Development and Programming Forums on MSDN.