Well Paul, try this one!
At the beginning, I haven't had this problem at all, now it's getting more and more frequent (or so it seems to me).
I'm working with WinXP, as stated in the other thread as well. I have two user accounts for myself - one as "admin" and one as "user", i.e. with restricted rights. The latter is the account I normally use. Sometimes I have to switch between the two, though. In these cases, it
can happen that I'm logged in with both accounts at the same time, finish whatever I'm doing as "admin" (usually update all kinds of programs), click "log out" and instead of the "Welcome screen", all I get to see is darkness, with my screen seemingly going into standby immediately. If I had music running (as "user"), it doesn't stop, but the PC doesn't seem to react to any kind of input.
I have tried to log in (using up/down and then entering my password), to shut it down in every way I could think of, but nothing happened until I pushed the reset button. After that, Windows started as usual.
What appears odd is that the only cause of such screen darkness that I know of is some problem with an overheating graphics card, or asking the screen to do something it technically can't.
Obviously, this is nothing too bad, but I still wouldn't know how to fix it. TIA for your assistance!
Plus it's good practice for me 
What I may not do, no program may do, which can't be too bad.