Thanks to all who helped me out with this problem. It seems to be sorted out now.
I downloaded the debugging tools to read the minidump files, but they didn't give any specific information.
What gave me a clue eventually was a single BSOD which indicated Power-Driver-State-Failure, so I started looking at possible corrupted drivers.
I didn't have any log-off issues when using Safe Mode, so "other people's" drivers seemed like a likely culprit.
After downloading and installing the latest ATI Radeon graphics card driver, the log-offs immediately returned to normal (i.e. a few seconds to log off, and a few seconds to shut down). I haven't had any log-off problems since! Fingers crossed they don't come back....
Oh the relief.
