DELL PHOTO AIO PRINTER 944

I RECENTLY PURCHASED AN ACER THAT IS RUNNING VISTA HOME PREMIUM WHICH IS THE HOST COMPUTER FOR MY WIRED NETWORK.  THE SECOND COMPUTER IS A DELL OPTIPLEX RUNNING XP PROFESSIONAL.  I HAVE MY DSL, THE DELL COMPUTER PLUGGED INTO A LINKSEY 604 ROUTER.  THE PRINTER IS CONNECTED TO THE HOST COMPUTER VIA USB.  MY PROBLEM IS THIS- THE DELL COMPUTER RUNNING XP CAN PRINT TO THE ALL IN ONE PRINTER BUT WHEN YOU TRY TO OPEN THE ALL IN ONE CENTER TO SCAN OR COPY IT GIVES THIS MESSAGE "THE DELL PHOTO AIO PRINTER 944 PRINTER DRIVER IS NOT INSTALLED. PLEASE REINSTALL YOUR PRINTER DRIVER"  AFTER YOU CLICK OKAY THE MAIN SCREEN OF THE ALL IN ONE CENTER OPENS UP BUT YOU CANNOT USE IT WITHOUT GETTING THIS SAME MESSAGE.  THEN WHEN YOU CLOSE THE AIO WINDOW THE MESSAGE APPEARS AGAIN & IT TAKES AT LEAST 6-7 TIMES TO CLOSE OUT THE MESSAGE.  CAN ANYONE TELL ME WHAT I AM DOING WRONG?  EVERYTHING IS SET UP WITH FILE & PRINTER SHARING & ALL THAT JAZZ BUT I HAVE NEVER WORKED WITH VISTA SO I AM NOT FAMILIAR WITH IT AT ALL & I AM JUST ABOUT READY TO SCREAM FROM THE FRUSTRATION.