When you are looking at msconfig, you will see items common to both logins. And you will see items specific to just the logged in user.
In the startup tab, look at the LOCATION column.
Items starting "HKLM" are common to both all users.
Items starting "HKCU" are just for the currently logged in user.
Items in "e:\Users\number_1\appdata\roaming\microsoft\windows\start menu\programs\startup" are just for the currently logged in user.
Personally, as a test, I would disable EVERYTHING in the startup tab.
Then swap to SERVICES and tick the "hide Microsoft services" box and the disable EVERY non-microsoft service.
Restart the machine in this state. How does Winamp behave?
If Winamp is now fixed, re-enable the services one by one and reboot.
Then re-enable the Startup items one by one and reboot.
Each time do a test with Winamp. See which breaks it.
If disabling everything didn't make any difference to Winamp... then the hunt is still on.