Update:
I was able to successfully install Civil 3D 2007 on a Windows XP 64 Bit machine (no, they won't let me format and reinstall the 32 bit os, but I tried
), but it took a little more work than usual.
A couple of problems I had to overcome were:
Directx 9 wouldn't install from the deployment because the version shipped on the Autocad DVD/CD doesn't support XP 64. This causes a dll not found error for D3DX9_27.DLL when you start Autocad. To get around this, I downloaded Directx 9.0C from and installed it before trying to install Autocad.
Also, because of pathing issues that caused multiple arx load errors while loading the program, I had to move the installation path out of Program Files (I used C:\Civil 3d\ for the path). I think it has something to do with the parentheses in "...Program Files (x86)...", but I'm not sure.
So far so good, but we'll see...
Caution - From what I have read, Autocad isn't supported on the Windows XP 64 platform, so installing on XP 64 is at your own risk.
HTH