Seems it wasn't IIS 7.5 Express at all, but the Web Deployment Agent Service.
After stopping it, I was able to start the site in IIS 5.1.
But thanks @taspeotis for your answer.
After doing some more research into why the Web Deployment Agent Service is on port 80, I came across this:
With a specific mention to:
If you install Web Deploy by using the
Windows UI, the Web Deploy remote
service will listen on
Windows XP, you will have to change
either the default port for the agent
service, or change the port for the
default Web site. On later versions of
Windows, the Web Deploy remote service
will not conflict with IIS on port 80,
and IIS and the Web Deploy remote
service can run at the same time. Web
Deploy and IIS both use the HTTP.SYS
URL registrations to make reservations
in the URL namespace. To install Web
Deploy with a different remote service
URL, see the Installing from the
Command Line section in this topic.