this is actually from dell support website.. hope it will correct your problem...
1 Ensure the correct resolution is set for the monitor. Flat panel monitors display the sharpest, clearest images when set to their optimal resolution. Sometimes games and other programs will change the resolution then fail to set it back once they are closed. This will then make the desktop look big, distorted, and grainy. Perform the following steps to adjust the resolution:
Right-click an empty area of the desktop, and then click Properties. (Or, click Start, click Control Panel, and then double-click Display.)
In the Display Properties window, click the Settings tab.
Under Screen resolution, move the slider to the desired setting. For most flat panels the optimal setting will be all the way to the right.
Click Apply, and then click OK.
If prompted, restart the computer.
NOTE: If the resolution will not adjust to the appropriate settings, the video driver may need to be reinstalled or updated.
2 Ensure the screen is clean. Finger prints and smudges will make the screen look blurry. Clean the screen with a soft lint free cloth. If needed, dampen the cloth with warm water or use a LCD cleaning kit that can be purchased from most electronics stores.
Use the On Screen Display (OSD) on the monitor to select the Auto Adjust feature. Selecting this will synch the video signal and can sharpen the video image.
Reseat the video signal cable to ensure a secure connection.
Use the On Screen Display (OSD) on the monitor to select the Factory Reset feature.
Test the monitor on another computer or test another monitor on this computer. If available, testing different hardware will isolate if the problem stays with the computer or monitor