W7 refuses to install-error msg: "Can't retreive Disk Info"

Doing an upgrade on a Vista32 machine. This machine has worked fine except that I can't install Vista SP2. I thought I would just wait till W7.
At very beginning of W7 install, get error msg: "Windows Could not retrieve information about the disks on this computer." Vista Explorer sees disks just fine. So does BIOS.
W7 just won't install. "Virtual Disk System" is turned on automatically upon boot, so this is not problem.
Any ideas? Can I force an install? Still do this upgrade?