Well, basically, most applications as well as games cannot use more than one core. So basically what happens is when an application that doesn't support dual cores get loaded, only one of the two cores get used up, that equates to about 50%, which seems to tally with your question.
Like ZOldDude said, try Orthos. If it goes 100% there, your computer is working correctly, if not, then perhaps there's a problem somewhere.
Correct. And you can cofigure Task Manager to show each core individually for processor utilization.
It's quite normal for a program to only hit one core and as a result you may have one core with high utilization and the other with low utilization.
If you can't get TaskMgr to show both cores seperate, then you may have an issue.