You can move the folder for that company from your C drive to the desired location on the server. If you're not sure which folder to move, open that company and go to Maintain > Company Information. There is a greyed out field that shows you where the company is stored. Be sure to close Peachtree before moving the files.

The reason the company was created on your C drive is that Peachtree has been installed on your computer as if it was a stand-alone copy instead of as a network installation. Unless this is a laptop and you use Peachtree away from the office, I would recommend that, in addition to moving the data, you uninstall Peachtree and reinstall it making sure you correctly answer the questions about where the data will be stored. Then new companies will automatically be created in the right location.