That doesn't sound like such a good idea. The winforms common controls are for the most part not thread-safe, and giving each control it's own process seems icky.
If you want to try this, perhaps give each form it's own process, or even just it's own thread. Or perhaps if you have a tab control then just give each tab its own thread.
Update
.Net provides something called an AppDomain that you can use. It's more than a thread, but less than a process. If you have a from with multiple tab pages, you could create a custom control that holds the contents for each tab page and put each custom control in it's own assembly. Then those assemblies can be dynamically loaded into their own AppDomains. From there you should be able to create an instance of the control you want. However, there are certain rules about talking across appdomains, so I don't know that you'd be able to just add that control to a tab page on your form.
