Download free ViewPower UPS Management Software
You can download a recent version of ViewPower (no need to have a serial number in order to install it) that works on Windows without problems.
In any case you can try and see if it works for your Mustek UPS.
To install it you need first to uninstall any ViewPower software you might already use. If you don’t have a backup of this, you may prefer to avoid the upgrade, unless you current version doesn’t work anyway.
Here is the order you should follow: first use the Windows Task Manager to terminate UPSAgent, then uninstall View Power, then install this version of ViewPower.