Big Problem!

Have you ruled out any problems with the monitor?
You didnt get hit with the blaster worm or some other virus did you?
Could be a power supply problem.

Hi. I don't have an easy answer for you, but here are a number of things to try. I don't know how computer savvy you are, so forgive me if some of these seem too bonehead to you.

1: First, disconnect all devices except the keyboard and take out one of the RAM sticks so you only have one installed. Disconnect power as well to all devices - except the mb, of course. See if mb boots and if you can get into the bios. If not, switch RAM. If that still doesn't work, you may have a corrupted Bios or a bad mb. You may be able to flash the Bios if it's bad - look to your mb documentation for that.

If you can get into the Bios, load defaults and save/exit. If you are able to boot 2 or 3 times and there are no errors on POST, then turn it off and hook up a floppy drive. Turn back on and boot from a boot disk, if you have one. Then attach the hard drive and boot from it. Keep doing this until you have everything reconnected. Make sure you try to boot at least twice after every change. When you are finished reconnecting everything and if the system is still stable, put in the rest of your RAM.

By the way, does your other system also have differently timed RAM sticks? Usually, it is best to keep the same timing RAM sticks even though the faster RAM should drop down in speed.
Also, turn cpu off before every change and be very certain you are putting the power connectors back on correctly as well as the IDE/floppy cables. Systems can hang from cables being backwards and you can fry your hard drive if the power connector is reversed. Generally, the red wire of the power connector goes on the inside while the yellow one is on the outside - nowadays it should only fit one way anyway. The IDE and floppy connectors usually need to have Pin 1 closest to the power connector. Pin 1 is often denoted by a colored line or dashes on the cable itself.

2: If the prior tips don't work, try the boot with a known good video card and repeat the above process. I know you said you tried another video card, but was it an old one? Did you try the viceo card from the other system? Do you have a PCI card that you could try?

3: Are these new systems actually upgrades? If so, your power supply might not be powerful enough to handle everything if you didn't buy a new one. How many watts is it rated for?

These steps should cover most hardware problems you could be having. If you only get problems once you hit the hard drive and Windows begins to boot, you probably have some software conflict. One thing to try is making sure your monitor's refresh rate is not set too high. See if you can enter Windows in Safe mode. Usually, that entails pressing the F8 function key just as Windows begins to boot. You can keep pressing it in the beginning of the boot until the menu comes up. If Safe Mode works, then you know it's probably a software setting that is the problem. If your monitor's refresh rate is set too high, you can change it in the display properties for your video card adapter.

Hope any of this helps. Good luck!