I had the same problem and just found the cause: too many entries in the PATH environment variable. Win7 (and most likely older versions as well) limit this variable to 2048 characters. If the variable gets larger this will cause problems. Try the following:
If this returns an empty string, your path variable might be to long. To fix this you will have to manually edit the path variable. To this, open the dialog to edit environment variables:
c:\windows\system32\systempropertiesadvanced.exe -> "Environment Variables"
Search for the "PATH" variable in the lower list (System variables), open it and make sure it has less than 2048 character.
This can be done by copying the value to an editor and
- removing redundant directories in the string,
- shorten directory path(s) by abbreviating them with variables.
After editing, copy the new path-variable to the "Edit System Variable" box. Then, after restarting Matlab, your problems are (hopefully) gone.