Cannot access internet without alternating restore/undo rest

I'm running XP Media Center 2005 SP2 on a HP Pavilion with Athlon 64 and dial up PCI K56flex data/fax SoftModem with Smart CP (Conexant HSF). About a week after loading Amazon cloud player, I could not get online. On my ISP login screen I'd get the normal SIGNING IN . . . DIALING . . . (modem transmission noise) . . . AUTHENTICATING . . . YOU ARE CONNECTED, where it then sits for a couple minutes, then goes back to SIGNING IN (ATTEMPT 2). Ultimately, it tells me COULD NOT DETECT CONNECTION TO THE INTERNET; but the line is still open. Diagnostics say the modem is o.k..(though a couple fishy things show in the modem logs). I suspect software got changed somewhere (?) because if I revert back to a restore point, I can get online (though it is a longer than normal sign-in). If I log off, but sign back in without powering down, it still works. But if I power down, the next attempted sign-in is unsuccessful. Then I can undo the last restore point, and I'm working again. So far I've been bouncing between the same restore point, reverting back to it and undoing it to get online each time! I do power down every day. I'm thinking of uninstalling the Amazon cloud player, but this seems like something is getting changed at each start-up. Any ideas on how I can identify the "bug" and surgically remove it without any major restores or reloads? Something in my startups/registry? Thanks!!!