Installed a new videocard today.
Plugged in the powersupply as directed (since when do videocards need powersupply anyway...).
Turned on the PC, start MSFS2004, marvel at the graphics.
5 minutes later my screen goes black and the PC boots itself.
Try again, same thing.
Turn it off, check everything again, find that one of the leads from the powercable isn't properly seated.
Reseat the lead, reconnect the cable, and all is well.
So if something seems right but still your system doesn't work: check your cables!