Just cause 2 game lags at higest resolution

LOL nubs

The game lags because your computer is weak. The CPU is old and weak in comparison to what has been released in the past 5 years and the 9400GT is not really meant for games it's more for media.

Your PC fails to meet the minimum requirements for Battlefield 3:

Minimum requirements for Battlefield 3

OS: Windows Vista or Windows 7
Processor: Core 2 Duo 2.4 GHz or Althon X2 2.7 GHz
RAM: 2GB
Graphic card: DirectX 10 or 11 compatible Nvidia or AMD ATI card, ATI Radeon 3870 or higher, Nvidia GeForce 8800 GT or higher.
Graphics card memory: 512 MB
Sound card: DirectX compatible sound card
Hard drive: 15 GB for disc version or 10 GB for digital version


http://bf3blog.com/battlefield-3-system-requirements/

Your PC barely meet the minimum system requirements for Just Cause 2.

Minimum Requirements

Operating System: Microsoft Windows Vista or Windows 7 (Windows XP is unsupported)
Processor: Dual-core CPU with SSE3 (Athlon 64 X2 4200 / Pentium D 3GHz)
Graphics Card: Nvidia Geforce 8800 Series / ATI Radeon HD 2600 Pro with 256MB memory or equivalent DX10 card with 256MB memory
Memory: 2GB RAM
DirectX: Microsoft DirectX 10
Hard Drive: 10GB of free drive space
Optical Drive: DVD-ROM drive
Sound Card: 100% DirectX 10 compatible sound card
Internet Connection: Internet connection required for product activation
Input: Keyboard and mouse (Microsoft Xbox 360 controller optional)

http://www.cinemablend.com/games/Just-Cause-2-System-Re...

Yes, you should be glad the game is playable at all. Both your CPU and video card are very poor for current games.

I lol'd. Saying what we're all thinking...

but yeah, OP needs a new PC.