Big Problem - Need Help!

First off, I run Garry’s Mod in a window because my cursor doesn’t show up otherwise. That may help later. The real problem is, whenever I try to open a dropbox (i.e. NPC Weapon), It just closes the spawn menu.
Please Help!!!

I had the same problem.
Try to use a different keyboard or try to edit your keyboard settings in Configuration Center.
It’s not a GMod related problem.

Where is the Config Center?
In the Control Panel?