7900gs sli windows 7 problem

[SpudMaster]

A friend has recently given me an Alienware m9700 laptop with 2 geforce 7900gs graphics cards in sli. Problem is I can only get it to run in a single gpu configuration. I select sli in the nvidia control panel, im then asked to restart, once its restarted it goes back to single gpu mode. Both gpus are detected under gpu-z.

Can anyone shed some light or know of a fix? The laptop is running Windows 7 64bit.

Thanks