This head-scratcher dates back as far as I can remember, and even Windows 7 hasn't fully resolved the issue. It's usually the result of a communication problem between the computer and the printer, and the culprit could be anything from a corrupted driver to a faulty USB cable to a wonky network setting (if you're using a network-connected printer).
Fortunately, you can send stuck print jobs packing. Download and run the Stalled Printer Repair app, which serves the singular (and singularly awesome) function of purging stuck print jobs from the queue. Purging them means those documents will never reach your printer--but after you resolve the issue, you can just send them again.
(For more solutions to perplexing PC problems, see "The 21 Greatest PC Mysteries--Solved!")