Well into controlled assessments now and gotta say my bespoke system detailed in this thread:

http://www.edugeek.net/forums/how-do-you-do/58143-controlled-assessment-how-we-did.html

is working like a dream. Although re-reading the thread I realise I made it sound more complicated that it is.

For the record it requires only ONE active directory account to function.

In practice, the whole thing is foolproof! and the only bother we get is teachers asking what the password is. When they've all finished I'll backup the folders onto DVD and delete them from the one user area. I had to write some stuff on our documentation about it, first time I had to generate some user codes I couldn't remember where I'd put the files.