Just came across a cool utility that deserves a mention called ProcessTamer. This baby sits in the tray and monitors processes that hijack the CPU and automatically sets the CPU to a lower priority. While this may not be ideal when you want the CPU to be used to its max potential (Eg. encoding videos) it's quite cool for normal use.
Take a look at it Here. The guys behind that rely on donations to deliver their software, so support their cause if you find it useful:-)
Process Tamer is a tiny (100k) and efficient utility for Microsoft Windows XP/2K/NT 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.

screen shot from developers