This sounds very much like a problem I had with my Vista laptop a few months back. It drove me crazy and it took me a long time to find the answer.

Aside from raising the dpi to the non-custom but still larger range (the bottom of the two choices on the dpi page), I also had to lower screen resolution by clicking on Start> Control Panel> Personalization> Display Settings> Resolution and then pushing the slide to the left. Just keep checking to see to see just how low you need to make it that makes the windows small enough for you to be able to access any buttons in the lower parts.

Hope this helps.
Kat