alt key doesn't work (not imac specific)

There is no Alt key on a Apple keyboard. Are you referring to the option key?

The standard white Apple USB keyboard usually has two items on the
Option key, a small alt on the upper-left and option spelled out.

If you need to engage an Alt key to work, depending on the OS X and
if you are also using a multiple button mouse, you may have alternative
options in the matter.

But, if you have both Alt and Option buttons, and the Alt won't work, you
may have to consider another method of making that button do its job.
Whatever that is. I've never seen any function occur at all with Alt.

• How-to: Get the ALT key to function properly under Mac OS X...
http://www.inkscapeforum.com/viewtopic.php?t=800&f=5

Since the 1990s the Option key has appeared on Apple keyboards;
there is an image of an Alt key with only a symbol of Opt on it, here:
http://en.wikipedia.org/wiki/Option_key

+My Keyboard Viewer does not show any changes, anywhere, when+
+I try to engage Alt separately from Option; but they are on one key.+

There is a way to enable the Alt key to work or perform correctly, but
the use of it appears conditional and purposed to specific uses. It
seems to me, the Alt key is functional in my iBook G4; I don't need it.

There is an alt+click discussed in several found search results; and in
• Mac OS X keyboard shortcuts:
http://support.apple.com/kb/ht1343

Maybe this will be somewhat helpful, or just curiously alternative.
Under what circumstances would the alt key be engaged, and
generally what are the expected results in normal use?

Several topics, depending on how they are approached, make me feel
ignorant. For the most part, I prefer to be well-read and act stupid.

Good luck & happy computing! 🙂

{edited}

If there is no way to get those two keys to function, be sure to
see about narrowing the issue to just the keyboard; in that a
replacement keyboard (even a good condition, second hand)
is one way to know for certain there is nothing else going odd.

The problem could easily be some circuit inside the keyboard
may have finally failed. You could see if you could assign a
different key to work as Alt (or Option) since the those keys
with Alt and +a symbol+ on them, it refers to Option, too.

See image of *Alt + Option 'symbol'* on same key:
http://upload.wikimedia.org/wikipedia/commons/thumb/5/51/Option-key.jpg/150px-Op tion-key.jpg

Spare mice; there are times when troubleshooting the unit
depends on having healthy peripherals and input devices.

You can use Keyboard Viewer & a Mouse to click on Alt/Opt.

After living well away from the urban drift, and where there are
more chances something made somewhere else could fail, I
try to keep a few spare parts such as those around; and I do
check second-hand stores or ads in a paper (increasingly rare)
where someone is selling compatible Apple computer items.
A few have been a very good opportunity to keep using Macs.

Almost any older Apple USB keyboard should work with your
computer; the later new metallic ones are a bit different and
usually require a newer OS X version and a specific update.
So, instead of that, I'm using a nice $10. white Apple keyboard.

Hopefully you can find at least a loaner keyboard to test the
computer; and if you can figure out the PC keyface differences
you could get by in an inconvenient state with a PC USB model
keyboard if not too complex, and it should work without drivers.

To make the use of a PC keyboard with a Mac probably more
complex, you could read about how to remap the PC model
for use with a Mac. Or find other reading on what keys do a
variety of slightly different things, maybe? Many keys are the
same, just a few are not. Several PC USB keyboard can do.

And those with Opt should do the job; unless you are really
talking about the need to use the Alt key in a non-OPT way.

Swap key function positions in PC keyboard to be like Mac:

• DoubleCommand (pre-OS X 10.4 version) freeware:
http://doublecommand.sourceforge.net/download.html

• How to remap a PC keyboard in Mac OS X:
http://hubpages.com/hub/How-to-remap-PC-keyboard-in-Mac-OS-X

Good luck & happy computing! 🙂

+{edited to add confusing links}+