I have checked the ports and all are free
That can't be with XP Pro, as it comes pre-installed with Windows IIS Server, which uses Port 80 - the same port as a default XAMPP install.

Search on your PC for a file called 'hosts' or look in the Windows folder and see if you can find it there. You'll find that Port 80 is assigned to IIS. You can either reassign it to XAMPP or uninstall XAMPP and do a custom install on, for example, Port 8080.

Additionally, if you have Skype installed you have to set Skype not to use Port 80.

Vger