The Astra 4100 is not compatible with Windows 7 since there is no Windows 7 driver for it and Umax has ended support for that model. You cannot find a link to the driver because it does not exist.
If it connects to a USB port you may find that software called VueScan
will allow you to use that scanner on Windows 7. Download the trial version here: http://www.hamrick.com/
If it works with VueScan you can then either buy VueScan or buy a new scanner instead.