I've done this before for a client. It was a couple of years ago, so I hope you'll excuse me if I don't remember all of the details off the top of my head.
You need to set up the vmware system to have two virtual machines running Windows 2003 and whatever other virtual machines that you would have in the 'real' configuration that you're trying to test. For example one machine 'in front' of the 2k3 cluster to host IIS, and then one or more 'client' machines, perhaps with various different OSs.
One detail I do remember is that each machine in the cluster needs to have several IP addresses configured in a particular way for the clustering to work correctly, but also that that was easy to do in vmware - just check out the MS clustering documention on technet or whatever your favourite source is.
I hope this helps
PS When I did this, the single machine that was the physical host for the vmware was a relatively high spec server. You may have problems if your single host machine is not very powerful or has limited memory. Remember, all the virtual machines have to share the physical memory on the host. The vmware dashboard gives you a good insight into what's happening while the virtual machines are actually running. Good luck.