In your case, I think the best software solution is to physically connect the Mac to the monitor, and using Remote Desktop to connect to the PC. You'll have very little display lag and almost zero input lag, compared to VNC.
As an added benefit, you can resize your Remote Desktop window to sit side-by-side with a window on your Mac, so you won't lose context when switching between Mac & Windows.
The only downside is that you'll have to crawl under the desk and swap the DVI cable if the Windows machine fails to boot when something goes wrong. That said, at my office we have a test machine that we've been using exclusively via Remote Desktop/VNC for about 3 or 4 years.
Every machine I use is running in a VM. I connect to the Windows machines via Remote Desktop, and I connect to the Linux machines via the VMware Remote Console.