From the FAQ:
Q.: I have Windows Vista Ultimate 32-bits and want to migrate to Windows 7 Ultimate 64-bits. Do I need to do a Clean install or can I perform an in-place upgrade?
A.: A Clean Install is required when migrating from 32 to 64 bits or vice-versa, independent of the initial and target versions,.
I did a custom install to switch from my 32-bit Vista Business to 64-bit Windows 7 Professional. Everything I've ever read, including the FAQ quoted above, says a clean install is the only way for that to happen.
I did that, yet it somehow retained all of my old files (mostly in a windows.old folder, with some remaining on the C: drive). How is this even possible?
I'm truly baffled.
-- BTW, I love this site. I found it while searching for Windows 7 on Google.