It is probably disabled using policies. If you can run poledit you can probably unlock it. But you'll probably need admin privileges for that as well. If you have permissions to manipulate the C\WINNT\LOGON.SCR you can grant yourself temporary admin privileges by making a backup of LOGON.SCR, copying CMD.EXE over it, log off, wait 15 minutes, a cmd prompt will pop up, run regedit or poledit to your liking, exit, log on, switch back the original LOGON.SCR.