Sometimes, you might encounter a problem where you can't use the 'Save As...' feature. The problem will repeat even though you reinstall Photoshop.
This problem might occur due to the screen resolution of your computers. Photoshop CS5 works properly in the screen resolution of 1024 * 768 or more. But some small mobile devices such as netbooks support 1024 * 600 only, which triggers this problem.
To resolve this error, the screen resolution must be adjusted to 1024 * 768. In Windows XP, it can be easily changed. However, it's not easy to change it in Windows 7. In Windows 7, please change the value of "Display1_DownScalingSupported" entry to "0" from "1" in the Registry Editor. Then, you will be able to change the screen resolution to 1024 * 768.