You might try running a system file check.
Open an administrative command window and type the following:
SFC /scannow
Hit enter and wait for it to finish. If the Explorer.exe file is corrupted or missing, it should be replaced.
If you use Computer or Disk Management to open the folders and go to \Windows, the explorer.exe utility is there. It should be 2,803 KB (32 bit version) , and you should be able to double click it to get it to run.
If it is not correct, there may be a virus at work. Are you having any problems starting Win 7? And yes, there is a registry situation but that usually causes login or startup issues. It can be fixed also, if the original cause has been taken care of.