Memory parity error after windows7 install

Hi I have an older optiplex gx280 desktop PC that ran great with xp pro. I decided to upgrade to Windows 7 and then the fun began.I came to the conclusion that Mrs windows doesn't like old Mr Dell. after I finally got win7 running I get the blue screen with the memory parity error, the system has halted message. I was running the original 512 mb DDR2 pc3200 333mhz memory stick with a 1gb DDR2 PC 5300 667mhz stick. Xp ran good with this win7did not so out with the 512 and replaced with another 1 gb stick like the other one and I a still getting the memory parity error after the PC has been up and running a while.

Any ideas?

Thanks