If you don't know the password of your Trend Micro OfficeScan client, you can still do the uninstallation by changing a value in the registry.

1. As an elevated user, start up regedit.
2. Navigate to the key HKEY_LOCAL_MACHINE\SOFTWARE\TrendMicro\PC-cillinNTCorp\CurrentVersion\Misc.
3. Find the value Allow Uninstall and set it to 1.

You should be able to uninstall the client without a password prompt.