First, check for your php.ini file, and if you don't have one, try to reinstall PHP then you'll have one created. Normally at that point, php will work properly. If not, then modify the short_open_tag and asp_tags values, and defintely make sure engine=on is listed in the same section.
; This directive determines whether or not PHP will recognize code between
; <? and ?> tags as PHP source which should be processed as such. It is
; generally recommended that <?php and ?> should be used and that this feature
; should be disabled, as enabling it may result in issues when generating XML
; documents, however this remains supported for backward compatibility reasons.
; Note that this directive does not control the <?= shorthand tag, which can be
; used regardless of this directive.
; Default Value: On
; Development Value: Off
; Production Value: Off
short_open_tag = Off
; Allow ASP-style <% %> tags.
asp_tags = Off
If apache still stops and starts, then either php5_module requires more memory than what your system has left, or its damaged.
You may also want to change the memory_limit setting. In most cases a value of 32M is ok.
Also, check the error logs and make sure error reporting is turned on. With a fresh installation of PHP, many options are explained in the php.ini file.
Another option is to check your system for disk corruption, and possibly reinstall both apache and PHP to rule out any damages to the files themselves especially if apache reports a segmentation fault. If other tasks on your computer abnormally freeze (especially the windows startup) then you might want to replace your RAM (memory chips).