First of all, please try this: press CTL+ALT+DEL then click file and new task. then type explorer.exe and press ENTER.
If this doesn't work, then it may be a virus causing registry troubles. If this is the case, then get through the following instructions:
1. Start Windows XP, after starting press Ctrl+Shift+Esc or Ctrl+Alt+Del to launch task manager.
2. Click File menu and select "New Tasks (run...)"
3. Now type CMD and click OK, this will open the Command Prompt.
4. Type following command on command prompt:
Reg add "HKLM\SYSTEM\CurrentControlSet\Services\RpcSs" /v ObjectName /t REG_SZ /d LocalSystem and press enter.
Good luck and if the problem doesn't get solved, please hit back for further help.