It could well be that you have not inserted the RAM firmly enough in the memory socket.
If this doesn't work, as Jim suggests, take out the new RAM and restore the status quo.
Boot up using a Windows StartUp disk (you do have one don't you?); at the Command Line prompt you should enter:
(despite what has been said elsewhere in these forums you don't require a space after ScanReg).
You should then see the Registry Checker, which will allow you to select the backup line that you know to be sound from the five listed; the system will restore to that point and you should be back where you started.
If you want to increase the number of previous Registry Checker backups, edit the SCANREG.INI file in C:\Windows by changing the number in the line:
(change X to the number you require)
Then Save and Exit this file.