I'm wondering if anybody could help me to connect to tor behind a proxy when just port 80 is open.
If you scroll down to the "Firewall Ports" option in the Tor manual, you can see that you can set it to "80" and this will only use port 80.
Here is the Tor Manual: https://www.torproject.org/docs/tor-manual.html.en
On Linux this config file, where you can set FirewallPorts among other things, will usually be in /etc/tor/torrc
On Windows, I'm not sure what you would do.