Windows has this irritating bug/undocumented feature/problem that if certain combinations of high memory settings (usually the more esoteric ones like buffershigh, fileshigh and fcbshigh) are present in your config.sys, it will hang on the 'windows is now restarting' prompt. This will also prevent the machine from displaying the 'it is now safe to restart the computer' screen, or switching off if it's an atx machine. It can also manifest itself as the machine hanging on the 'windows is now shutting down' screen

There are two solutions, listed in reverse order of discovery:

Work through your config.sys, commenting out lines until the problem goes away.

Patch win.com to say 'windows is not restarting'. Not really very helpful, but a lot more honest...