Microsoft Remote Desktop client for Mac. Free, bulletproof and seamless, I use it professionally almost daily for client servers all across North America without issue.
This is, of course, assuming you have a direct IP connection to the PC, and you've set it up to accept remote connections. If it's on a private network, you'll need a compatible VPN client as well (the Cisco client can be had for free if you google around a bit. Look for uni websites for a download).