Researchers at the SANS Institute's Internet Storm Center estimate that an unprotected PC will be compromised within 20 minutes of being connected to the Internet, down from an estimated 40 minutes last year.

The estimate is based on observations of vacant IP addresses, which received reports approximately every 20 minutes.

According to the researchers, if those reports come from Internet worms, the unprotected machine would likely become infected within 20 minutes, which is especially troublesome because most patches that would protect the computer take longer than that to download and install.

Scott Conti, network operations manager for the University of Massachusetts at Amherst, said that, as a test, his institution recently put two unprotected computers on the school's network, and both were compromised within 20 minutes.