Can't change properties of default monitor in Windows Xp

I had installed Windows Xp as my operating system along with a Gefore 7900GT which is been used in order to send the graphics to a CRT monitor. Now the problem is that, the picture keep on flashing and the monitor goes blank for a 100-500ms at a time at random intervals, let say every 1-30 seconds. I had tried to change the monitor from "default monitor" to the correct one, but couldn't succeed in it. Can any body tell me that how do i get rid of the above issue? Does any body knows about it? Kindly help me out to resolve the above issue.

I think you need to install the video drivers on ypour system. Probably the Gefore 7900GT drivers too on your pc. You will get the latest drivers on the Gefore 7900GT official website.

I think you can try out booting into safe mode and removing the default monitor and see whether you get the default monitor properties back on your system.

Try to check out your BIOS.I think there may be an option to set the BIOS for an external monitor as default/primary display. The bios setup menu should able to display your default monitor on your system. If you want you can drag the Taskbar from one screen to another in an extended desktop setup by right clicking on some free space on the Taskbar and making sure there is no tick next to the 'lock the taskbar' setting and simple drag it over to the other screen keeping cursor towards the bottom and drop it there.