Glad to hear you found the problem so quickly.
I was going to suggest that sometimes, rather than going through all of the great troubleshooting and 'cleaning' suggestions above that it is sometimes just easier to back up the data, format and start fresh and clean.
I like to do this at least every 2nd year or so and I find I always end up with a much faster machine than when I try to 'clean it up'. I keep a pretty tight ship via regular usage of the above techniques anyways but I find a clean start is still the best medicine after a while.
Depends on your usage, habits and fear of the format command though. It's not for everyone.
