WIFI Doesn't Connect

We just purchased a Dell 1520 using Windows XP.  We have a Dell Wireless 1390 WLAN mini-card installed in the computer for WIFI access.  On home wireless networks or at WIFI hotspots, the networks are detected, but then fade out and back in again repeatedly, preventing it from connecting.  We went online and reinstalled the correct driver, but no luck.  We have also tried disabling and then reenabling but that doesn't work.  Can anyone offer any advice?