You should be able to connect two separate PC's to the monitor and view the output from one or the other by selecting either the digital (DVI) or the analog (VGA) input selection available at the monitor controls. I'm not familiar with your particular monitor so I do not know how to change the input selection exactly, but you should be able to make the selection through the monitor's buttons on the front, or through the OSD menu. Here is a link to Using and Adjusting Your Monitor
Which input are you using on the problem computer, DVI or VGA? Have you checked the connections too ensure they are secure. Additionally check that the video cables are not bent sharply or pinched. Two PC's connected at once should not be a problem, but when you test the monitor use just one PC on at a time.
Both DVI and VGA should give good results, with full screen. I see from the link I gave you above the OSD menu has a Factory Reset selection, you might want to try that if the second PC is not full screen. Also, I see a Custom Scaling under the Image Control Menu Option as well.
Let us know what you find out with the second computer attached.