I've found that the biggest culprit for taking up port 80 on newer Windows installs is the BranchCache Service (#3) in this list...
SQL Server Reporting Services
Web Deployment Agent Service
BranchCache
World Wide Web Publishing Service
These 4 service probably cover 90% of the native Windows Services that take up port 80.
The other 10% is the hidden HTTP.sys service/driver which takes port 80 when another service requests it. Run this to disable it, and reboot.
sc config http start= disabled
Aside from Skype, TeamViewer is also very commonly installed software, and will take port 80 if not configured otherwise.
List taken from: Opening Up Port 80 For Apache to Use On Windows