click here to test your memory as you appear to have a problem with your RAM. It may just need checking to ensure that it is inserted properly which is very easy to do. If it is properly in place and the test shows up a problem then it is best to get some from click here as their Memory Selector will help you pick the right memory for your PC. There turn around is very quick and anything ordered tonight you should have by Wednesday.
If you have more than one memory stick then test each one independantly to see which is faulty but remember that there should always be a stick in Rimm 1 on your motherboard so a bit of switching around is required.
Hope this helps.