I work for a small charity and after an audit of our IT system we have
realised we need to purchase further Office 2000 licenses. We have not
be able to find anywhere that stocks them and have been told they are
not available. Does this mean I do not need to buy licenses?
Additionally, this has led me to wonder why I am buying licenses
anyway. If I did not, how would anybody ever know? And what would
happen if they did?

Microsoft has charity licenses available,
http://www.microsoft.com/licensing/programs/open/opencharity.mspx

Note that, generally speaking, purchasing licenses of the latest
version of Office (xp/2003) does usually allow licensing for any
previous version (only one licensed product per license, though).

Non availability of old licenses does not preclude your obligation to
obtain licenses for product you do not have licenses for.

If you do not buy licences, you are in violation of the EULA, and if
found out (most likely by a disgruntled former employee who may tell
the Business Software Alliance http://www.bsa.org/ or Microsoft
directly), the penalties to return to compliance can be severe
(several thousand dollars per violation).

If you wish to avoid all that, you may also wish to consider a
significantly less expensive option (read: free to use and copy
legally) such as www.openoffice.org.