Please open a command prompt (admin) like this: Getting to a Command Prompt (CMD)

Type: "diskpart"

You will notice your prompt changed.

Type "list vol" to list any available disks.

Your Windows disk will probably be the biggest NTFS partition you have. (The file system is listed under Fs)

Select the disk you would like to mount using "select vol <number>".

Now, type "assign letter=<letter>"