For years I've been disappointed with the Windows process scheduler. It is so easy for a process to suddenly take up all of the CPU and completely "freeze" your PC for minutes at a time.
Fair scheduling is something high-end UNIX variants like Solaris have dealt with successfully for a while now, while Linux is still coming to terms with.
An interesting freeware tool I've been trying out recently is ProcessTamer, a Windows utility 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. Now my PC doesn't become completely unresponsive for long periods of time, but on the downside some applications like Firefox take longer to start up.
Sad that I have to consider using tools like ProcessTamer, but so far it has done a pretty decent job.