boofhead
As you have found out, these types of problems can be a swine to resolve.
All I can suggest is the process of elimination and to make sure:
No viruses - full virus scan
No trojans - try "The Cleaner"
No spyware messing with your connection - Adaware
(I just found a new one for me - a module of Coreldraw (Attune) is listed as spyware and right enough, Adaware picked it up).
Have a look at
Interactive Modem Troubleshooter - it may help.
Let us know how you get on.
Best!
HA