First, get the patch that allows d3d9.dll to work in SA-MP
You can get it from MadMax's old post, here:
[link]Next (important
), if you currently have your GTA at max resolution please lower it down 'cause if you will run windowed mode in your max resolution a big window will just fill your screen
(800x600 is recommended for people who run it at 1280x1024 at max
)
Lastly, download this:
[link]Place the custom d3d9.dll in your SA-MP dir and your done!
(If you want to revert back to fullscreen just remove/rename the d3d9.dll from your dir
)