My friend had a similar problem.
He would start up AC, get decent rate for about a second and then drop to 2-3 FPS.
It turned out that Windows Update had installed the (essentially useless) generic Windows driver over the proper ATI driver.
The driver was up-to-date but it was the wrong one. You should be able to check this in the device manager or something (I'm not exactly sure; it's been a while since I used XP).
To fix, just update your driver software (IIRC, Nvidia provide a fairly easy-to-use installer for their drivers).
He would start up AC, get decent rate for about a second and then drop to 2-3 FPS.
It turned out that Windows Update had installed the (essentially useless) generic Windows driver over the proper ATI driver.
The driver was up-to-date but it was the wrong one. You should be able to check this in the device manager or something (I'm not exactly sure; it's been a while since I used XP).
To fix, just update your driver software (IIRC, Nvidia provide a fairly easy-to-use installer for their drivers).