Windows 7: random/frequent BSOD on AMD 6400

Hi - I have been having repeat BSOD or reboots - usually within 10-15 min, sometimes after several hours. Variety of messaes.

System - AMD 6400+, 2 GB RAM, NVIDIA GEFORCE 8800 GTS, ASUS M2N-E SLI
About 2.5 years old.
Installed Win 7 six months ago.

Started crashing after la
st set of security patches about a month ago, restore did not help. getting more frequent.

Startup repair from computer not working, repair from orig install disk helps briefly.

Last messages:
0x00000050 (0xFFFFFFFFFFFFFFFF, 0x00000000000000008, 0xFFFFFFFFFFFFFFFFFF, 0x00000000000000000)

Previous two messages
0x0000007E (0xFFFFFFFFC0000005, 0xFFFFF8000c354707, 0xFFFFFF88009A8FB8, 0xFFFFF8800009A8810).

Now getting BSOD upon trying to start from original Install disk - stops at mid load and yields
0x00000019 ()x00000000000000020, 0xFFFF8A000109790, 0xFFFFF8A000109820m 0x000000000509010E)

System crashes on starting from install disk and says

BAD_Pool_HEADER