Explorer.exe - Application error

What service pack are you currently running? If you can, get the machine to boot up in safe mode. If that is possible, get SP4 on that machine. Hopefully you have more than one computer that you can d/l SP4 and put it on a cd.
If you can't get into safe mode. I guess its time for a re-install

If the workstation will start in safe mode, you may have a driver conflict. From safe mode, disable as many of your third party drivers as possible. Boot normally. One by one, enable your drivers, rebooting after each. If you come across the error again after enabling a driver you will have your cause and can try updating the driver. Make sure to keep track of what drivers your have enabled as you repeat this process.

If safe mode does not work, try repairing from the original cd.
If safe mode deos work, then look in device manager for any conflicts. you can also look through the event viewer for more hints as to the cause, also update the service pack.