What I would recommend is setting the VM to a non-persistent hard drive once you have it configured the way you want your 'base system' to look like. I did this with several machines for demo purposes so that if the sales guys fouled the demos system up, all I had to do was shut the system down and restart to go back to a 'day zero' configuration.
In the VM's configuration, once your vm configured where you want it to 'restore' to, power down the vm and open the settings. On the hard disk settings under 'Mode' select "Independent" then "Non persistent". Doing this you will most likely have to delete the snapshot you've already taken but this will allow you to run the VM and all of the changes to the system will be stored in a separate temporary disk file that is deleted when the VM is powered off.
If you wish to update your vm base image later, simply power down, change the mode to Persistent then power it up and make your changes. Once changes are done, power down and repeat the to set it back to Non-persistent.
There's a lot of good documentation out there but I highly recommend the VMware documentation center:
I know you are on 5.1.0 but the disk modes have been the same for a while now. Here is a direct link to disk modes with brief explanations: