Fast user switching

[LineCutter]

I can't find it on the net & Microsoft don't seem able to make it clear:

It must be possible for Windows to know if the Welcome screen is on display - presumably be setting a key in the registry. I've almost done the "Computer use Time Limit" program, but it seems a little harsh to penalize users by counting time in use by another user as being theirs. I can't see where to find a marker that I could use to suspend counting though

Any ideas?