Windows grinds to a halt within minutes of starting

Over the past few weeks, the performance of my computer
has become progressively worse, to the point where I get
error messages about not having enough memory and am
forced to reboot just a few minutes after starting
windows. I have been running with two stacks of 128 mb of
ram (for a total of 256). The funny thing is that it
seems to run just as well with only 128 megs plugged in
as it does with 256 plugged in. Also, I recently tried a
new 512 meg memory card and the computer gave the memory
error messages just as fast. So windows is having memory
problems seemingly regardless of how much or what kind of
ram I have plugged in. Can I get a little help?

Thanks,

Andy