I don't do this everyday so, guess what, I always forget how to do it and find myself spending lots of time searching for how to do it. This is a note so that I hopefully will know where to look when I forget it next time.
Hint one: Don't get lost in the massive amount of pages about setting the registry keys for activating logging. On XP Tracing is used instead. You activate it like this using a shell and from the command prompt you type:
>netsh netsh> ras netsh ras>set tracing PPP enabled netsh ras>exit
The log is found in %SYSTEMROOT%\tracing\PPP.LOG.
You disable it the other way around. So from a command prompt you type:
>netsh netsh> ras netsh ras>set tracing PPP disabled netsh ras>exit