"Have installed Spoeedtouch (twice)...... "
Did you READ and understand the installation directions?
Do you install it correctly?
"You install part of the software for the Speedtouch 330 BEFORE you plug it into a USB port, then you install the rest of it."
If you didn't do that correctly, you MUST unplug it's USB connection, un-install it's software in Add/Remove Programs, then install it properly.
NOTE what I pointed about to about USB ports. The Speedtouch probably won't install properly unless it is detected properly when you plug in the USB connection during the middle of the software installation.
NOTE that, same as for any software installation, you need to have a minimum of programs running when you install software, in case they interfere with the installation, including you may need to DISABLE the resident part (the part that scans all the time) of any anti-malware software that is loaded.
"...my ISP reckons PPPoA is required for using their service..."
Did you look to see if the ISP has instructions regarding that? If they're the one who supplied you with the Speedtouch 330, they may have specific instructions for that model, or they may have supplied printed instructions, or an ISPsCD that helps you with that, with it. .
According to the manual I found the Speedtouch 330 supports that automatically. Unless your manual says different, so does yours (sometimes the features and the modem manual is different depending on which region you bought it in). If you don't have the proper manual, find it on the web, for the region you bought it in.
If you were using a router, you set up the type of internet connection in the router's configuration, in an internet browser via a URL that accesses that, not in Windows (other informing it than you're using a broadband connection). In most cases the default connection type is Dynamic, and you must change that if you need to connect some other way.