Whenever I launch a DirectX app and put it in full screen it always changes the resolution too 800x600 and I can't get it too 1280x800 unless the app has a resolution changing option

How do I fix that and here's my laptop specs
ATI mobility Radeon HD 4200 128mb VRAM
Driver version: 11.2
DirectX version: 11.0
CPU: AMD Turion II dual-core mobile M520
Model: HP dv4-2145dx