There is a potential for system problems if you change your setting and there is no "undo"! 
You must correct the problem!  

Setting your screen resolution: Either the Display icon from the Control Panel or 

1. 

Right clicking a blank area of the desktop, then select Properties.  

 2. 

When the Display Properties opens, select the Settings tab.

3. 

Once in the settings tab, change this slider to 800 by 600 (or more if desired) then click OK.

If you change this setting and Windows prompts you to reboot, you may have a problem.  After rebooting, you receive an error that there is a problem with your resolution setting, it is very likely you have the wrong (or a bad) video driver.  

Perhaps the first place to look for the correct drivers is at the computer manufactures support site.  If their site does not have the drivers, (beside regretting you bough one of their computers), try this:

Go to the Control Panel then the System Icon.  When that opens, click the Device tab then locate the Display setting.  Click the plus sign next to Display and record the video adapter information.

Hopefully you have a "name-brand" video adapter, e.g. 3DLabs, ATI, Intel, Matrox, Nvidia, S3, SiS, Trident, etc. 
Note: The above are links to those sites.

If so, go to their site and locate the correct drivers and installation information under their support or downloads page.  Install the driver and try changing the settings again. 

If you have NumberNine, 3DFX or another adapter that is no longer supported, you may still find drivers someplace. 
Try searching Google!

More information and help with video issues try: