Replaced motherboard battery, now scr...

Turned laptop on, beeped and said rtc battery flat.
fitted a new one, reset current date and time, laptop works fine, only thing is now the screen display is too big!
I can't reach down to the start menu or taskbar unless I hook an external monitor up, that works fine, and displays correctly.
Even the default acer start up and windows boot up screens are too large on the laptop.
It's almost as if the laptop thinks it has a 14inch screen to fill up
instead of a 12 inch one.
it would appear that a setting in the memory of the motherboard was lost when the battery went flat. Where is the screen size information stored on the motherboard?.
There are no user changeable screen settings in the bios menu.
The laptop came with a rescue disk and a driver disk, but neither of those changed anything.
I tried an updated graphics card driver too but that didn't change anything either.
The laptop originally had win98se, but I fitted a new hard drive, 512 mb of memory, and did a fresh xp sp2 installation 2 years ago, using the drivers and files provided on the acer website, and up till now all has been good.