Establish a VPN connection in System Preferences > Network. Then get a copy of Symantec pcAnywhere 12.0 or greater. (Version 12.0 introduced a Mac-compatible client.) Install the office PC as a host, and your Mac Pro as a client. Alternately, you could set up a small Bootcamp partition, load XP and use its built-in Terminal Services client. You'd have to set your office PC up as a Terminal Services host for this to work.
UltraVNC on the Windows PC at work, just make sure you have a static ip address and ports 5800 & 5900 are open and forwarded on your router. I use chicken of the vnc on my macbook and it works great. Remote desktop works as well. UltraVNC & Chicken of the VNC are free and work fantastic.
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).
It's so much easier than all of that. Go to logmein.com. It's free. There is a pay version, but the free account is more than adequate. You install a small client utility on the PC and the Mac and then you can pull up your PC desktop in your browser on the Mac, and vice-versa.
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I use Microsoft's free Remote Desktop on my Mac to connect to my PC at work. You don't even need a static IP address as stated above. If you have a dynamic IP address at work and a router that supports dynamic DNS (which most do), then just setup your router with a free account at dyndns.org and never again worry about your IP address. Works great.