keyboard problems

Im running vista home pemium on a dell xps 410 and everything was working
fine until i installed the new windows updates (4/9) and shutdown. When i
first tried to turn it back on it was just a black screen, the diagnostic
lights showed a faulty memory module and my owners manual suggested
uninstalling the ram one at a time and reinstalling to test it. I took one
out and restarted and it loaded fine, I put the other back in and this time
it loaded fine. I go to log in and my keyboard(usb) doesnt work. I tried it
in all other ports and it still doesnt work. I tried my other usb devices in
the same ports and they work fine. I went out and bought another keyboard and
it has the same problem. Both keboards work before start up but not after.
Any advice would be much appreciated ty.