problem with display adaptor

I use Windows NT 4.0 workstation. a few days back, the system crashed and so i need to re-run the OS. i had an old CD of NT4.0, but with service pack 1. so i installed the OS, but presently i had the problem with the display adptor. the resolution is now set to 800×600 with standard VGA adaptor with 16 bit color. the monitor i use is SONY SFI and from my previous knowledge, i used to have better resolution. i tried to change the present resolution, but to no avail. the system refuses to change the resolution. so i request any of you to suggest me to restore my screen resolution.