Wireless routers (and wired ones, and switches, except for ethernet hubs, but let's not go off on a tangent) do not allow you to monitor the traffic being sent and received by other users on your local network. The reason for this should be fairly obvious: it would present a huge security issue. I assume you are using some sort of wireless security; short of "cracking" that wireless (which is not only difficult, but could also be construed as an invasion of privacy if these tenants are not of a familial relation to you), there is no way for you to detect from your ordinary computer how much traffic is being used by each computer on the network.
In order to determine this, you will need to install a customizable router firmware such as OpenWRT or dd-wrt on your Linksys WRT54G. Fortunately, this router is renowned for being very customizable, and there are a great variety of available firmware packages. The router itself is able to see all the traffic flowing in and out of it (it has to be able to see it, or it wouldn't be able to route it), so you would just have to use a traffic inspection / monitoring software, which is a well known, and solved, problem.
The answer to your original question of whether there is Windows software that does this (with the assumption being that you wouldn't also modify your router) is No.
However, if you expand your search to "what program could I run on a custom wrt54g firmware in order to do this", then your question essentially becomes a duplicate of this SuperUser question, which also links to an Engadget article on how to do this with snmp and MRTG.
The general feel of it is this:
- Install custom firmware (e.g. OpenWRT) on your router.
- Install MRTG on your computer. You can use Windows if you have Perl installed, per the instructions, or you can install it on Ubuntu (arguably easier if you have a running Ubuntu install).
- Set up your router to listen on the snmp.
- Connect MRTG to the snmp server on your router.
- Monitor and enjoy.
If MRTG doesn't pan out for you, an alternative, WallWatcher, which was announced as no longer supported but will probably still work on your old router (wrt54g is very old :)), is available here.