Kind of resolved. Deleting all the WA files allows DW CS5 to boot without the WA Memento error, and everything else worked fine.
I then purchased the FormAssist upgrade (and another extension). Guess what? The FormAssist extension **ALSO** crashes the Extension Manager, and puts me back at square one.
I'd normally just pass on the extension and move on, but I used the darned thing in CS4 to validate most of my pages. Guess that'll teach me. So, I have to solve this.
Given that no one else in the forums seems to have experienced this issue (and I can't be the only one running CS5 on a Win7 64-bit box), I'm guessing something went wrong with my CS5 installation.
Now uninstalling, deleting all CS5 traces, and then reinstalling CS5. God only knows if this will work, or if I'm wasting yet more time.
I really wish my old computer hadn't died. Upgrading is a PITA.