I tried using OpenVPN instead of the VPN client in Windows to access my computer in school, which Internet connection passes through a proxy. It works, however, I could only connect using my laptop which has OpenVPN (set to connect at TCP port 80, instead of 1723). Will it be possible to change the connection port in Windows 7, so that I can connect to my PC at home through any Windows 7 PC in school? Thanks.

Have you tried changing the port? That is up to your client whether or not you can change it unless you are using the client that came with Windows 7.