Cant have more then one person logged on

for some reason my computer just all the sudden stopped letting more then one person be logged on to windows at a time so now we have to totally log off the computer to let someone else on instead of just switch user...please let me know if u can fix this thanks

One of the accounts may be corrupted. I would try unticked fast uers switch and reboot....then try selecting it again and see if it works. It sounds like there is a problem in your registry and there is little I can think of to fix that other than running -:

sfc /scannow

Copy and paste that command into your "run" box.....have your XP disk ready, and after doing this, go to win updates and restore anything that MSoft says needs updating.

Hope that is of some help.
~A