explorer.exe won't run? That's weird. Does explorer.exe just not run, or does an actual error message pop up saying explorer.exe cannot be found, or explorer.exe cannot run or something?
if it just doesn't run, then you should be presented with a blank screen. Believe it or not, it is actually still possible to run things here, you just won't have any desktop icons or task bar etc.
If you hold down the windows key on the keyboard and press 'R', then a little box should pop up letting you run programs. Type in "regedit" (without the quotes). This will bring up the registry editor. Be VERY, VERY, VERY CAREFUL HERE!!! Do NOT change anything unnecessarily, because you can really mess up your system.
Go to: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon (the registry is structured like a directory - click on the + to view what's inside the directory, and click the folder to select it - so click + next to HKEY_LOCAL_MACHINE, then click + next to SOFTWARE etc.. untill you see Winlogon, then click on that).
In the main section, there should be a few things displayed. One of them is called "Shell". Next to this it should read "Explorer.exe" (without quotes). If it doesn't, then double-click on it to bring up another box, and type this in. close the registry and reboot (you should be able to hold Alt and press F4 after shutting the registry and it will give you the option to shutdown, reboot etc. - I believe this works without explorer.exe).
If the problem is that it can't find/run explorer.exe and comes up with an error, or the registry already has explorer.exe in as the Shell or resetting the Shell doesn't work, then your problem is indeed a strange one. If it can't find explorer.exe then it is possible to replace this. Just copy C:\Windows\explorer.exe from a working machine to the same place on the not working machine (Win Key + E brings up a file browser, but this may well not work if explorer.exe isn't running). This is a bit of a long-shot, but it might work.
Otherwise, it is possible to do a REPAIR-install. This will not wipe out any data (although I still recommend backing up first if you can). If you still have your Windows disk, you can put it in and boot it up as though you're gonna re-install, but it should find the existing windows installation and give you an option to repair install it (not repair, because this brings up a command line. It should be repair install - do not go through with any operation that involves reformatting or selecting drives, as this means it will reinstall, completely destroying everything you have on the drive). This can be a bit tempremental, but it might work.
If none of that works, it looks like you'll have to reinstall
I suppose you could try Microsoft Tech Support, but they're usually pretty useless.
