How to inject code into a process?

Originally Posted by jonwil

I am looking for a way to inject code into an already running process [...]

Here some article that should help you. Hopefully you can code in C(++)!

Take a look at this article, it's one of the best I found...
Three Ways to Inject Your Code into Another Process
http://www.codeproject.com/threads/winspy.asp

Also interesting (not read)
Extending Task Manager with DLL Injection
http://www.codeproject.com/threads/taskex.asp

Good luck...