Thanks. It appears that Windows 7 has a service called http.sys which starts automatically and uses port 80. To get around this (without hacking the registry and deleting stuff) you just need to install XAMPP and set Apache HTTP to run as a service using the XAMPP Control Panel. Once you've done this reboot your PC. When your PC restarts, the XAMPP Apache HTTP service starts before http.sys and is able to grab port 80.
If anyone's interested I found the solution online at [edit]link removed[/edit]. If the "Start As a Service" option doesn't work for you, there are some other things you can try.
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