Alt+Space and PrBoom+

I often make Alt+Space (strafe+use) in Doom, but PrBoom+ drop the window menu, and that is annoying (especially when recording demos). How can I disable it?

That's a Windows shortcut that applies to all application windows by default. Short of a recompilation with the PrBoom+ application specifically made to ignore the shortcut, I don't think it can be done specifically for prBoom+.

If you don't mind disabling it for all applications and using an external utility, check out this SU post:

http://superuser.com/questions/61069/how-can-i-disable-selected-windows-7-keyboard-shortcuts