There could be quite a few things going on here, one of which is the fact that your PSU might be fried, CPU dead, GFX card dead etc.
One thing to make sure is that all of the power cables are connected to the motherboar in the correct fashion and that all of the cables in the PC are in their right place.
If this doesnt help did the computer start up until recently? What exactly is the problem? Does it start every now and then or not at all?
15'' Macbook Pro
Intel Core 2 Duo 2.16ghz
1GB DDR2 SDRAM
Windows XP/Mac OSX 10.5
No longer a Guru of games...