Things can be a little funky with mobile GPUs, because of manufacturer's tendencies to modify the device IDs.
The latest drivers at support.dell.com for the M90 are for Vista. They may work for Windows 7.
Microsoft Update also has Windows 7 drivers for the 2500M, dated 3/6/09. The nVidia 195.62 drivers posted above are newer, but I don't see the 2500m explicitly in the supported products list.