connecting Ubuntu PCs via network cable
 

How can I connect two PCs having Ubuntu OS with network cable?

Similarly how can I connect two PCs one having Ubuntu and another having WINDOWS OS?

You mean, aside from just plugging the cable in?? When you say "connect", can we assume you mean how to set things up, so the systems can share files between them, or do you mean connect as in physical connection?

If you mean you want to share files, there are types of sharing, but you don't say what you want. If you mean just being able to copy a file back and forth, you can use SSH or FTP. If you mean a shared drive, for Linux-to-Linux, NFS is the best way to go. Between Windows and Linux, use Samba.

Without details, it's hard to say more, but there are many how-to guides about setting up NFS and Samba to follow, that you can find on Google.

I want to connect two PCs via cable to transfer files.

Right. We got that from your first post.

Again, how do you want to do it? You've got many options to suit many different needs, but you provide no details. In my first post, I mentioned SSH/SCP, FTP, Samba, and NFS. Did you read that?