OK here goes. i just got more ram for my pc.
went from 2 512mb pc 3200 sticks to 2 1gb pc3200 sticks (corsair Valueram)
used Unbutu 7.04 memtest and ran it overnight. no errors.
so i rebooted into debian, checked mail and went to bed.The next morning i went downstairs and the pc was frozen on a screensaver. I rebooted and it happened again only during the screensavers.
I disabled the Screensaver and it has stopped. When i boot into windows xp pro it starts to load and just reboots.

Is it bad ram or just a power supply going bad?
any ideas?

Have you peaked into the bios to see if it autodetects the ram?
I doubt it's either of these. Another thing you can try is to disable the guick post in the bios. Most newer computers will have the bios perform a quick Power On Self Test without testing all components. By disabling the quick post the bios will check the state of all components before passing control to the MBR of the boot-able drive. If there's a problem, it will either give a beep code where you count the beeps and intervals and check the on-line mother board documentation to see what it means, or a voice recording or it will write the error message to the screen.

Did the memtest report the right amount of RAM? Or was it just testing the old memory chip? If the computer worked just fine with the old RAM chip, then it must be the new one that's the problem. You could try another RAM bank, but most likely the chip just does not work well, even if it passes the memtest.

Have a look at the specs of the RAM and what your BIOS autodetects.They get it wrong sometimes....

mem test posted the correct amount of ram.
i am starting to think it may be a heat issue. i have recently started finishing my basement and i put a wall that my pc desk is behind. now it is not getting as much airflow as it used to. I put the side back on the case to see if the contained air pressure resolves the issue. I have also checked the bios and i have disabled the quick post. We will see if this solves my issue. thanks for the prompt response.