Whenever I try to play DVDs through WinDVD it either plays them, but only the sound, no picture or it brings up an error message saying something along the lines of 'overlay failed adjust colour depth or screen resolution'
I have tried adjusting them, but it doesn't work.
Since I'm hopeless with computers, does anyone have any idea how to fix this??

Not having WinDVD I can't supply an answer. However, I have done a bit of 'Googling' on the matter and it seems that others are getting the same the error message. Unfortunately, no one comes up with a definite answer to it.

Type the words: overlay failed windvd into Google and look into the top entry that comes up. You will find at least some information on the problem. This is the best I can come up with for now. More information might be found by using different keywords for the search.

*EDIT* Having the latest version of DirectX might make a difference. You could go to Windows Update to see if the version you have needs updating. (Current version is 9.0b).