Does anyone know why Windows Defender takes much longer to scan with Vista than with XP?

Windows Defender usually runs silently in the background on Vista, when your computer is idle. Vista being so depended on "all" of it's RAM to operate...programs such as Windows defender gets low priority. But 3 hrs. seems extreme unless your system's memory is almost maxed out. Have you been using Windows Update regularly? You may not have all of Windows Defender's updates.