Windows Boards

Recently I cannot get past the logon prompt in my Windows XP.  The PC boots up OK until the prompt comes up "Log On To Windows" and askes for Username and Password.  I input my correct Username and Pass and then it appears to freeze up.  Sometimes it just asks again instead.

The only change I have made recently is I took out one of my RAM sticks (I had 2 in there) to use in a different computer.  I know the RAM works OK because it is the same stick as before.  I didnt think a memory downgrade would cause this problem.

Please help.