If it doesn't even complete POST, sounds like motherboard or memory issue. To troubleshoot memory very basically, and assuming you have more than one stick in the machine, remove the memory sticks and swap them (slot 0 stick to slot 1 and so on). Make sure they're seated tightly and click into place. Some folks are surprised by how much pressure you need to apply to seat a memory stick properly. Don't be afraid to manhandle it a bit. If you line it up right you won't break anything. Even with only one stick, just remove it and reseat it. If it still freezes up after reboot, it may be the motherboard and/or bad memory. While the box is open, check for obvious problem like loose power supply/ide connectors. And if its full of 5 years worth of dust, at the very least blow it out with a can or two of compressed air. At my shop I use a high power compressor to really clean the inards.