Oops. sorry.. brain fart.. haha..
Anyway, It is a process error... The ISAPI filter is not able to output because of cocomittent threading..
I assume you have WebMin? You should go into webmin (if it's running) at https://localhost:10000 ( or the other way you admin apache) and then set the isapi sit einto its own worker process threading model.
There is more info at apache.org on how to do this, or you can upgrade to apache 2.0 and that should make php 4.3.1 happy.
Cheers!