You could use it for plotting waypoints / routes / tracks, for transferring to a Garmin. So it may be of use for that. But no way of actually loading the maps onto the Etrex.
I've not tried the Anquet software myself, but most of this mapping software has its own quirks. It may take a while to get used to the interface etc.
There are a few other options for Windows software with OS maps. They may be cheaper, if you wanted to add more maps.