iTunes will not open

I'm having troubles opening up itunes. Every time i click on the icon I get
the following message
The folder "iTunes" cannot be found or created, and is required. The default
location for this folder is inside the My music folder.
I had itunes on my hard drive but it got too full and had to transfer the
library to an external hard drive. This worked for a while but suddenly
stopped working with the above message. iTunes is still in my program files
and all the files are still there on the external hard drive but the program
wont open.
When I tried to move the My Music folder back to the main hard drive, it
turned into Angus' Music - not My Music.
Can I restore the My Music folder somehow or does anyone know how to change
iTunes so it actually opens?
Can I change the options somehow??