press ctrl-alt-del goto File - new task - explorer.exe.
If it fails again then goto C:\WINDOWS\ServicePackFiles\i386 and run the explorer.exe file that is in that directory.
Reboot.
If explorer.exe continues to fail goto C:\WINDOWS\ServicePackFiles\i386 and copy the explorer.exe file over to your C:\WINDOWS directory Click yes to replace.
Reboot
Hopefully it will start normal now. If not you have bigger problems.