We removed the internal card and slid in a PC-Card or express card. Now I hear some squawk and squeal over that but I'm sharing the solution I've used dozens of times last year. It's cheap and gets it working.

http://www.geeks.com/details.asp?invtid=WPN511NAR while out of stock is what I used last time.

http://support.dell.com/support/edocs/systems/ins1521/en/om_en/pdf/RT722A02MR.pdf shows this model uses the newer express card.

I can't find a cheap Express card that shows 7 support so try http://www.airlink101.com/products/awll6075.php which is less than 20 at http://www.geeks.com/details.asp?invtid=AWLL6075&cat=NET

As to the Vista install, you may have forget the Dell QuickSet utility. Tell me you installed that on Vista and 7.
Bob