Ok, I am normally very capable when it comes to computers and software, but this one is starting to stump me. I recently downgraded my girlfriends Acer Aspire 3680 from Vista to XP as it became apparent that this notebook couldn't handle vista well; (on a side note, this is the second aspire 3680 that I have downgraded). For some reason no matter what I do the computer does not see the internal dial-up modem. Now, as its internal there is no installation disc, I have downloaded 6 different acer aspire 3680 modem drivers from Acer support centers and not a single one worked. In the Device manager, the modem drop down is not even there. I ran the modem troubleshooter and it detected no port was found, but Ports weren't in the device manager either. This one is really stumping me and i'm tired of looking at that screen. Any ideas?
Check for Unknown Devices as ATG mentioned, if there is nothing then you might want to check that the modem is turned on in the BIOS. I don't see how it could have turned off in the downgrade but checking shouldn't hurt.
Also, my 3680 uses "Agere Systems HDA Modem" and I downloaded that driver off the main Acer site. The link is here (European Acer Site) and the Agere Modem is down at the bottom of the XP drivers list. Have you tried that one?