Dont do windowed, instead, enter these commands:
-dxlevel 80
-autoconfig

if those dont solve your problem, change gmods res

if that dont work, heres the command for window:
-window
pretty easy to remember :)

Yo' welcome :)

What are your specs?
if u are putting a lot of addons u should eliminate those addons that wont work, or follow the valve graphics putting al the advanced grapics in the recommended for ur computer "* recomended to your computer"

-width 800 -height 600 -dxlevel 80 -console -window

thats acctually the one i used for CSS before

Remove all addons
i swear, this should work
if it doesnt.. do what the others said