Between Window and Full Screen

I don't think it's just me, and it could be a graphics/processor issue. Is there some requirement to be able to run window mode smoothly? Because apparently I lag more in window mode than in full screen. I know this is NOT the case for everyone, it's even reversed for some people, which leads to more confusion for me.

Apparently the game tends to burn more CPU when running in window mode, and it's evident when I show the FPS in game and it randomly jumps to 300 and I get fast to slow to fast etc. FPS jumps. When running in full screen, the FPS remains at a constant 60, which is perfectly smooth and fine. Now I do want to run window mode, but with the issues on it for me right now it's too inconvenient to use. If anyone has any advice on how to fix it, or even just a reason why it's not working the way it should, any and all help would be appreciated. What I don't want is people posting saying "well it works fine for me I dunno about you" without even the slightest hint of a pointer or advice.

Anyways here's a few specs of my laptop, which I bought about two years ago:
Model: HP Pavilion dv8301nr
Processor: AMD Turion 64 ML-34
Speed: 1.79 GHz
RAM: 2.00 GB (upgraded from 512 MB)
OS: Windows XP
Graphics: ATi RADEON XPRESS 200M