You need to go into Control Panel>System>Hardware>Device Manager and scroll to Display Adapters. Right click the adapter, go to Prpoerties and then click the Details tab. Scroll to HardwareIDs and take a note of the four characters which follow VEN for Vendor and also the four following DEV for Device.
Go to http://www.pcidatabase.com and input those characters in the two relevant boxes and follow the trail to the right driver. When you find the list of the Vendor's drivers, they aren't in order so use your browser's Find facility to track the Device details. With luck you'll have an executable file but some will be a zipped folder.