A writer for About.com heard similar tales:
With a little more digging, I learned that conkers are thought to repel spiders indoors. An old wives' tale, if you'll pardon the expression, asserts that you should place horse chestnuts on windowsills and in corners of your rooms to keep spiders from entering the house. The conkers are thought to contain some kind of oil or chemical that spiders detest.
I wondered if this could be true.
I've had a quick browse myself but can't find anything objective to confirm or deny.
Do horse chestnuts deter spiders?