If you having problem starting apache server on Windows 7 after installing the XAMPP, the problem is most likely that you already have a web server running.
Windows 7 comes with the IIS server and it's started up by default on many Windows 7 installations.
This is usually a very bad security risk, mainly because you should never have any server running that you don't need. Even more important, you should never have any server running that you are not aware of.
Anyway, on Windows 7 you should check if IIS is running and stop it if it is running.
Start -> Control Panel -> System and security -> Internet Information service.
You then look in the right panel to see if Start button is grey and stop button is green. This would indicate that IIS service is running. Just click on Stop button to stop it.
Then you should be able to start the Apache server from the XAMPP control panel