Now just click on that little purple flower next to "pe_glass_clear" it'll pop up with a prompt to select a new texture. Go to your MW texture directory (probably on your CD) and select tx_window_pane. You shouldn't need to repeat the process for the other door or whatever as they are probably linked.
I also edited the UV in my screenshot because otherwise that texture gives a funny dark line through the center of the door. Do you know how to edit the UV in nifskope?
Sweet, I got it to work. But you're right, there are dark lines down the middle. And why do the glass horizontal shelves disappear at certain angles? As seen in my picture:
http://img833.imageshack.us/img833/7919/96689115.jpg
I'm not too clear on understanding what UVs are or about editing them. I mean, I'm in the UV editor but I really have no clue what that window pane is for or what I can do with it.
http://img215.imageshack.us/img215/9210/25413421.jpg