Kill it, restart it? Symproxysvc (or whatever it is called, part of Norton AntiVirus) runs away from time to time and uses 99% CPU until I kill it. It then restarts itself and works fine ... well, until the next time it runs away.
If it's not a one-off, then i'd get rid of it (unless it's doing something useful, of course). Thrashing the hard disk doesn't sound like it can be a good thing.