RunWithAffinity is a simple tool that makes it easy to run programs on only one core of a multi-core CPU.
The program is essentially a shortcut wizard built into a single dialog. Choose your executable file, decide where the shortcut will be created, and in a click you can create a shortcut that will launch your target program on just a single processor core.
This will, of course, slow down whatever programs you choose to run this way. But it'll also mean they're unable to use all your system's CPU time. And so if you've a large number of background processes running on your system, then you could allocate them all to CPU core #1, say, and the other cores will remain free to run more important foreground applications.
RunWithAffinity may also be useful for troubleshooting purposes. Some old games, for instance, have problems running on multi-core CPUs. If you're having problems getting a game working, use RunWithAffinity to launch it on a single core and that just might help.
The Windows command line tool "Start" can already run programs on a selected core, and more flexibly, but RunWithAffinity is definitely much easier to use - and it can be a very handy optimisation and troubleshooting tool