[...] Sobig.F, a variant of an older worm, began spreading on Monday in Europe and has infected an estimated tens of thousands of Windows-based computers, said Patrick Hinojosa, chief technology officer at Panda Software, based in Madrid.

It arrives in e-mail and includes a variety of subject lines, including "Your details," "Thank you!," "Your application" and "Wicked screensaver." It has caused some corporate e-mail systems to grind to a halt, according to Sophos Inc.

When the .pif or .scr attachment is opened, Sobig.F infects the computer and sends itself on to other victims using a random e-mail address from the address book.

It also prepares the computer to receive orders and tries to download files from the Internet, said Hinojosa. It was unknown exactly what files they were, he said. [...]