In addtion, check all the connections very well. Also, since its very old, check the connections and switches at the back, there may be an on\off button in a weird place as well as on the front panel. Also make sure its set to 115 volts.
If no go then take out all the is\pci cards that you dont need to get the computer to boot. Unhook the secondary ide channel if the primary has your main boot drive.
Disable and unhook all you dont need to boot up.
Sometimes a bad floppy drive or a bad cdrom can prevent a computer from booting, even a bad mouse, so try with different keyboard and mouse if you can.
There is a recent thread here where the person did everything to get the computer to boot, when he replaced the mouse the computer booted to windows.
Good advice + great people = tek-tips