Windows users are been urged to make sure their systems are clean from an email worm which is programmed to overwrite user's files on 3 February. Blackworm (AKA Nyxem, MyWife or Tearec) has infected more than 300,000 systems worldwide, based on analysis of logs from counter web sites used by the worm.

Blackworm arrives as the infectious payload of email messages with spoofed sender addresses claiming to offer obscene pictures or pornographic movie clips. Subject lines used in the malicious emails include: The Best Videoclip Ever, Fw: SeX.mpg, Miss Lebanon 2006 and F***** Kama Sutra pics. The worm only affects Windows PCs.

If activated, Blackworm tries to disable security software. It also tries to harvest email addresses from infected PCs in a routine designed to draw up a hit list of targets for infection. Blackworm is programmed to download updates of its code onto infected PCs.

Its behaviour is little different from standard email worms apart from the fact it is programmed to overwrite DOC, XLS, MDB, MDE, PPT, PPS, ZIP, RAR, PDF, PSD and DMP on 3 February. The worm creates and opens a ZIP archive in the Windows system directory, potentially giving away its presence on infected systems but don't rely on this.