Strange keyboard behavior(s) on iMac

[Richard Bienvenu]

I recently had to replace the original (black) keyboard that came 
with my iMac, but something is amiss: I thought that the new keyboard 
(from Apple)  worked properly until I discovered that the sound 
control keys (top row, at the right) do not work, _nor_ can the 
right-hand option key on this new keyboard give one accents such as 
the umlaut. Oddly, the left option key can do this.

Here's where it gets even odder: that same new keyboard works 
perfectly on my eMac. I tried out two other keyboards that I knew to 
be in working order, but on the iMac these two would give me keyboard 
control of the sound level OR a working right-hand option key, but 
not both.

This suggests that something has happened to innards of the 
iMac--either hardware (?) or to the OS X.2 that is running on the 
machine. A saleskid (undergraduate) at my campus store suggested that 
I might want to bring the machine in to have it checked by one of 
their Apple-certified technicians.

Before I go to the trouble--and the expense--of taking the iMac in 
for service I'd like to know if anyone can suggest other things to 
try. I have restarted the iMac, but that didn't help. I switched the 
keyboard cable to the other USB port, with the same negative results. 
Re-installing OS X.2 might do the trick, but that's not entirely 
without risk, or am I wrong?

Thanks,

Richard Bienvenu
Boone County, Mo.