Too-tall taskbar cuts off the bottom of window display and forms/can't click OK, etc.

Try recalibrating/adjusting the display on your monitor. Most monitors now have an "auto adjust" feature that will properly center the image for you onscreen.
Change your computer's display settings:
Right-click on a blank part of the desktop somewhere and select Properties
In the Display Properties window, click on the Settings tab
Change the Screen Resolution one click to the left or right to increase/decrease you resolution by one level
Click Apply
When the new resolution appears, you will have 15 seconds to click on OK to confirm you approve the new settings...if you do nothing, the screen will revert back to the previous settings
When you have a satisfactory setting, click on OK

If not then install display drivers ! This should be included with the cd's shipped with your machine ! Or your M/B manufacture