Click Start, click Run, type: devmgmt.msc
Click the OK button. When the Device Manager opens scroll down to Display
Adapters, click the + to the left of that listing to expand, right click on
the display adapter below the listing, and left click Properties. It's
possible that if you upgraded to Windows XP, that Windows XP didn't have a
driver for your display adapter and used a generic driver. You should find
the info for the driver used by doing the above and then clicking on the
Driver tab. If there IS a Windows XP compatible driver for your Display
Adapter, go to the site of the maker, download it, and install it. Or, if
Windows XP DID install a display adapter driver for YOUR display adapter
(not a generic driver), go to the maker's site, and download and install the
latest Windows XP driver. The above is predicated on the fact that you know
how to change the resolution, and were unable to.