hello world script not working

Hello On a WIndows XP professional edition I am running Apache 1.3.27 with PHP 4.2.3. I am using this configuration to locally work on web projects before publishing them. Although most of the things work ok, there are some times that the server returns a 500 internal error. It happens with the same scripts all the time. The simplest script that returns this error is the hello world script! I can't say this is a rule, but usually it happens when the part of the script that is actually executed is quite small. For example, if I have a script that works ok and on the top of it I add: $dada = '1'; if ($dada) { echo 'Hello world'; } else { the rest of the script } then it returns an internal error again. I read about a bug in the apache's website. Are there any common experiences regarding this? Thanks.