with pc switched off, open the case and take out the ram memory stick then put it back in (reseat it) making sure that it is in properly with the clips at each end. Now switch the pc back on and start it up.
If it still won't work then you will need 'memtest86' click here to test if it is faulty.
(cut&pasted from another thread - credits to skeletal forum member)
"In essence, you need to download this and create a bootable floppy or CD. You can then run this outside of Windows which is more reliable. Basically, you don’t want anything else to run at the same time as the tester, because they could conflict and give misleading results."