Do I need to upgrade my server?

I have a very small computer network in the office. Currently I have a winxp machine acting as a file server for several other pcs. Basically it’s just for our accounting database. The other pcs are being slowly upgraded from winxp to win7 over the next few months. The server is behind a router firewall so there’s no way it can be accessed from outside the network. Can I keep it running winxp after support ends for that os next year? Or should I upgrade it to win7?