it's a real bug in the AT&T installer. i had the same problem quite a while ago with XP x64 and now with vista. i fixed it by doing this:
run the PMM installer. before you start the installation, but after it extracts the installers (i believe it is to a folder under %TEMP%), copy the AT&T .msi file to a new directory.
download/install a copy of Orca (an MSI editing tool, part of the free-but-huge Vista SDK but you can find a copy of the installer pretty easily).
look through all the launch/install conditions and remove all of the version checks. the installer requires the OS version is *exactly* WinXP x86. after editing it, save the MSI, then run it.