Fullscreen

[LeHott]

Hello, guys. I recently bought Assassin's Creed IV: Black Flag on.. Erhmmm... It's NOT Uplay ( It's uplay >_< ).
 The problem I have is, I've got quite a big PC screen which will only let me play games in fullscreen if I set the game resolution to 1,280 × 720... And due to my bad graphics card, I can only play it 100% lagg-free if I set it to 1152x 864, and it is quite annoying when I play it like this



NOTE: For some reason, I can play lagg-free on 1280x720 in Multiplayer, but not in Singleplayer aswell.

TL;DR is there an option ( a software or something ) that lets me play fullscreen with low resolution? I don't care if the image is going to be stretched, I just want it fullscreen >_<
MY MONITOR


( LG FLATRON 22en33S-B )