This should be simple to diagnose. I don't think it has anything to do with the changes to your computer, unless you have a dial-up modem in the computer that is connected to the phone line.
Unplug _everything_ that connects to the phone line from _all_ the wall sockets, including all extension phones, extension bells, fax machines, and all broadband filters or splitters, including the one for your broadband modem.
Plug a corded telephone into a wall phone socket and check for dial tone. Try several sockets, and if possible several corded phones, to be certain.
If you can't get a dial tone at all, it's extremely likely the problem is at the telephone exchange. I had that problem recently. The line from my house to the exchange was OK so the broadband was still working, but there was a fault in the part of the exchange that deals with the telephone, and I couldn't get a dial tone or make or receive phone calls.
If you get a dial tone in some cases, check each telephone wall socket independently, and re-plug the devices one at a time, and use a process of elimination to find out where the problem is.
Kris in Wellington, New Zealand
December 2009