Memory detected/Memory failure on Dell Dimension PC

I have a Dell Dimension 2400 PC. 
The other day I installed Vista Business successfully after playing a lot with the display driver for Intel 82845g on-board graphics. When I restarted the system it worked fine.  after the first few updates (including the infamous reliability update) and restarts, when I left the computer idle, and checked back later, it went into standy mode, with no display whatsoever on the monitor.
later i found out that the problem was with the memory modules/memory failure, since the diagnostic lights a and b were amber and c and d were green.
as suggested in the manual, I removed the RAM modules on there (512 mb) tried many combinations, including an older RAM module that came with the computer, with no success.
the diagnostic lights still glow the same way
here is my configuration
Intel pentium 4 1.6 gHz Intel on-board 82845 g display DDR RAM 512 mb (2 modules) chipset- Intel 845g series
wondering if somebody can provide a solution as I am out of all ideas at the moment

thanks in advance
:)