A reformat should always be the last option and I suspect that it is more likely to be a memory problem.
First of all check to make sure that it is properly in place by (with the PC disconnected from the power supply) opening up the PC tower and using the side clips to pop the RAM stick out and then reinsert it. If the PC still won't work then click here to test your memory. If it is properly in place and the test shows there to be 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.