Try reinstalling the driver from here. It might be more up-to-date. Have you also tried XP system restore (instructions here) to a time when it worked?
I have a inspiton 530 with XP provessional o..s.  I installed dual monitors so i could run  a software that requires dual monitors.  I am using a USB connected vga dual monitor adaptor.  All worked well for a while, but for some reason now both monitors display identical information.  Using windows control panel/display properties/settings,  monitor two is indicating "inactive".  I used tp be able to switch it from one monitor to dual monitor operation froma tool bar on  the software which needs two monitors, but it will no longer switch form single to dual monitor operation, Using device manager to test the usb connected monitor the test indicates that the divice is working correctly. Any helpful suggestions will be appreciated.  
dual monitors both the same