apache could not load, and start and stop

I'm using xampp to start the apache server, but it told my port 80 is in used and cannot be started.

This was caused by some other application/program taking the port 80. Some people uninstalled skype and work fine. Someone said uninstall the apache and working on the regedit and re-install and succeed, but it has too much works and risky. And I found some people were starting apache on a machine that has IIS running and didn't notice like me.

For me, I found a couple reasons that I started apache fail.
1. a dll doesn't exist in \ext\ folder that cannot be added in the php.ini (it's ok if it's commented)
2. I'm running PHP 5.3.8, apache 2.2.21, sqlsrv driver php_sqlsrv_53_ts_vc9 on windows 7. The version non thread safe (nts_vc9) of dll could not be loaded.
3. I disabled a service in SCM which is called "World Wide Web Publishing Service". I don't know why and when this service was installed, enabled and started automatically. Anyway, with the disable of this service, my apache starts running flawless!