|Actually, if you're running XP, I wouldn't bother with msconfig.|
I have used, and still use (on my XP machine) the following tool:
Startup Control Panel by Mike Lin
Click on the link, download and install it. Once installed, you'll find the applet named Startup in your Control Panel.
I said at the outset I figured this was something loading after bootup (based on the same deductive reasoning Paul employed) but you assured me nothing had changed. I should have responded with, "Nothing you know of......"
Regardless. You might try using Startup Control (or msconfig) to remove all unneccessary programs and applications from the startup sequence and see if that fixes the issue. If it does, enable one item at a time and reboot until you find the one that makes the network interface stop functioning.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.