How can I repair a blue screen error, Bad pool caller ?

I have a Dell Dimension 3000 with 512mb of Ram. I installed a single stick of ram because I thought one of the sockets was bad. It ran fine, but slow after that, so I thought I would try changing back to the old ram. Well now I get the Blue screen and an error message, Bad Pool Caller. I have tried everything to repair this, but nothing has worked so far. I can't boot up the computer in any mode. It gets as far as saying starting windows, then the Blue screen comes up with the bad pool caller message and locks up.

Can anyone help me to get my computer up and running again.

Mr Ed939
June 2008