I have been running 12.04 for some time. . .but the very last update of 12.04 has caused a "split screen" on my system.

The left half is about 2/3 the screen, and the right half is about 1/3 of the screen.

I can open a window up, and drag that window across my screen and make the right half disappear, but it shows up again.

It seems to turn the pixels off, until some other program/window turns it on.

Both spit screens contain my task bar, but only the left screen has activation mode of the task bar.

I have checked for updates, but it says all updates are "up to date".

Anything I can do?

sounds like a graphic card/driver related issue.
what's your graphic hardware and what driver or kernel module are you using?
there's some simple commands with some grepping that provide useful info, i don't remember exactly atm, but it's really just a duckduckgo-search away ;-)