Hard to say exactly why with so little information. If you are using a disk to install the scanner, check to make sure it actually has Vista drivers on it (and 64 bit drivers, if you are using 64bit vista).
If it doesn't, you have to see if the manufacturer offers vista drivers for your scanner on their website.
Lastly, if you really need help with a specific peripheral like this, you need to list a full make and model. 'pbell scanner' is pretty vague. I can deduce Packard Bell from that, but that's it.