To eliminate the possibility that the bat file is not working did you go the command prompt? Just type:
It will just list all the commands, but at least you will know its found by the system.
subst P: C:\ArmAWork\
Quotations are needed if there is no space in the file name.
If that still doesn't work, then find cmd on your start menu or in explorer, and right click -> Run As Administrator, and try again.
If that still doesn't work, there's got to be another way to mount a virtual drive. Look around for some other tool or try Mapping A Network Drive. You'll have to share the folder over the network, then just browse for you own computer when browsing for the network location to map.