Click Start, Run and enter cmd. This should open the command line interface of your windows operating system. The command that we need is the “subst” command and we use it the following way.
subst drive folderLets say you want to assign the drive letter X to the folder d:\movies on your hard drive. To do that you write the following command:
subst x: d:\moviesThat is all. Fairly easy isn’t it? Now the drive letter x: remains accessible as long as you do not turn off or reboot windows. If you want to make this permanent you will have to do the following.
Create a new file and name it drive.bat. Edit the file and add the line subst x: d:\movies to it and save it.
Right click the Start button, select Open All Users. Open Programs, Startup and right click that location. Select New Shortcut, and select the drive.bat file that you created. Select Next and finish. The command will be executed for all users with every startup of windows.
If you are running for example Windows 95 you could edit the autoexec.bat and add the line there.
Update: If you do not like to work with the command line or bat files, you could head over to NTwind to download Visual Subst instead, which is a gui version of the program. Source: http://www.ghacks.net