When companies release hardware like printers or scanners they also release DRIVERS that help the hardware work with an operating system like Windows XP.
If Microsoft, or anyone else, releases a new operating system it is up to the hardware manufacturer to update the driver for this new operating system.
If it is an old scanner then the company who made the hardware may not have bothered to make a driver for Vista or Windows 7.
You need to go to the company web siote and see if they have a Windows 7 driver for the scanner