It's probably not going to break much of anything on your local machine, assuming no Windows domain nor DHCP is involved.
However, the identifier 'localhost' specifically and always is defined to mean the local machine. This means that any other computer that wants to connect to you by name will be unable to do so, because attempting to connect to 'localhost' will be attempting to connect to themselves.
Similarly, it is entirely likely that your domain controller (if you have one) will either disallow the hostname or will encounter problems when registering your hostname. Similarly, if you used DHCP, you may encounter issues if the DHCP server is set up to auto-register your host's name.
Therefore, if it's the only computer you're using on a very simple network, you're probably OK. Anything with any level of complexity, and you really should choose a different name.