Nope, win.ini is nothing to do with it.
The only place there are details are in your client (remember Outlook is not the only email client). You normally have to add your username/password to the pop section but smtp is normally just authenticated through your connection via their adsl line
You could try thunderbird(
Mozilla | Firefox web browser & Thunderbird email client) to rule out issues with outlook but TBH it does sound more likely that the PO is at fault.
When you have the issue there is a geeky way to test where you open a telnet session to the POP server and you can issue commands directly on the server. If you feel competant enough to do this let me know and I'll give you the details. Its all Command line stuff but just involves typing stuff in