This MAY be an issue of an outdated video driver... so have you updated everything about your computer from Dell's site?
The video drivers are updated with the July 9th, 2010 version for the Nvidea GeForce GTS 240 card - so are reasonably up-to-date.
Work around found.
It looks like Premier doesn't support displays with text and other items set to 150% (used if your PC uses a large TV screen as a monitor). Switched the display to 125% and the buttons work.
On PRE7 if you set text to 125% all the dropdown menus disappear except for a solitary '>>' which acts like an ubermenu!
Insanity is hereditary, you get it from your children