Windows 7 limit the number of threads that a process creates

Well, talking about shotgun solutions, I think you should go the script way using, as an example Command Line Process Viewer/Killer/Suspender for Windows NT/2000/XP and write a script (any language accepted) that would take all active processes (excepeted the one of your choice) and set their affinity to any other processor than the chosen one. However, as it is only an affinity, I can't ensure you that your processor will be left alone for your preferred program.

Personnally, i consider the optimal solution to your problem is to set both affinity and priority.

A search over the net brings some programs that may help. They are all freeware.

BES - Battle Encoder Shirase

BES is a small tool which limits the CPU usage for a specified process: for instance, you can limit the CPU usage of a process which would use CPU 100%, down to 50% (or any percentage you like). With this, you can use other programs comfortably while doing something CPU-intensive in the background. By limiting the CPU load, you can also cool down your CPU immediately when it happens to get too hot. Of course the processing speed will slow down proportionally if you limit the CPU usage, but it should be much better than crashing because of heat or (in the worst scenario) having your computer broken with a burned CPU.

Process Tamer

Process Tamer is a tiny (140k) and super efficient utility for Microsoft Windows XP/2K/NT/Vista/Win7 that runs in your system tray and constantly monitors the cpu usage of other processes. When it sees a process that is overloading your cpu, it reduces the priority of that process temporarily, until its cpu usage returns to a reasonable level.

Process Lasso

Process Lasso is a unique new technology that will improve your PC's responsiveness and stability during periods of high CPU load. Windows, by design, allows programs to monopolize your CPU without restraint -- leading to freezes, hangs, and micro-lags. Process Lasso's ProBalance (Process Balance) technology intelligently adjusts the priorities of running programs so that badly behaved processes won't negatively impact the responsiveness of your PC.