a) Reboot the PC.
b) Once the computer is booting up press F8 or the function key that lets you to a menu which can start your computer in Safe Mode.
Among Safe mode there will be other options like
a) Safe Mode
b) Safe Mode with Command Prompt
c) Safe Mode with Networking.
d) Normal Start up
e) Last good known configuration.
Select a and while the computer is booting up in safe mode, immediately press F8 this will show advanced options. Some stupid way to get to advanced options.
You will see a screen that would allow you to start the computer in VGA Mode. With VGA mode the screen resolution is auto set to 640 X 480 which works on all monitors. This will start the computer in VGA mode.
Once the system is up and running do the following
a) Go to desktop
b) Right click , Properties.
c) In Properties select Settings tab and choose 800 X 600 or the last known resolution that worked on your monitor.
d) Click Advanced
e) In Compatibility section select Ask me before applying the new settings.
f) Press OK and your new resolution will be in safe mode.
Now you can reboot the computer and start in normal mode.