4gb of corsair 800 ram (only 3 are showing up in vista...grrr)
This is to be expected if you are running 32 bit vista Ultimate. You will need a 64 bit OS to use all 4 gig.
Another issue with RAM unrelated to the above: Some motherboards have a hard time with all slots populated with RAM. Try running only two sticks until you sort this out.
Since you have same problem with XP you have a hardware problem of some sort. Could be BIOS settings or could be hardware failure.
1. As suggested run memtest86 and try to isolate a possible bad ram
2. Try a different PSU
3. Check temps for overheating, back off any overclocked settings.
4. Check your RAM settings in BIOS. Speed, timings, and voltages. Be sure your RAM voltage has not dropped back to a default setting below what your Corsair needs.