The game is running slowly even on a good system.

I recently bought a DELL laptop and Heroes of Might and Magic V: Golden Edition, and it runs terribly, even though the first 5 minutes are fine. I tried setting the resolution to 800x600, but it still lags. I have DirectX 9 from November 2007, and the ATI drivers are also from November. My friend has a similar laptop, but with an NVIDIA 7300GO, and it runs fine.
I can play Oblivion on medium settings without any lag.
System: Win XP SP2
Processor: AMD Turion 64x2 1.8 GHz
RAM: 2 GB
Video card: ATI X1150 256 MB + 256 MB shared memory
Hard drive: 160 GB
Since I bought it recently, the system is new.