And since you are not using a support service I'll offer a test I use to sniff out if this is hardware or software.

But first, since this machine had DSL it's possible you had some DSL MODEM driver installed. I don't see where you removed that. Sorry I can not tell you how to. I've seen dozens of such PPPoE DSL software over the decades so I can't guess which method to use.

With that out of the way, I'm also hampered by the lack of a quoted "error message."

Moving onto the test I use. I boot a LiveCD called Ubuntu and see if I can get on the internet. No install needed, no learning Linux either.

More at ubuntu but for 99.99% of the time I boot the LiveCD and then pull up FireFox and I'm in. If there is hardware or windows software issues you have to tackle that.
http://davestechsupport.com/blog/2008/03/08/how-to-setup-internet-connections-in-ubuntu/

In parting I've lost count of posts like yours when the antivirus expires.
Bob