Network Associates's McAfee division has upgraded the threat level of a virus named Navidad from low to medium.

McAfee says the virus seems to have increased its rate of infection recently, although the malicious program actually contains a bug that allows you to render it inactive.

People receive Navidad via email in the form of an attachment named Navidad.exe. Correspondents receive the attachment as a reply message to an email sent to an infected user.

Once the virus has infected a PC, it prevents you from launching any programs of the .exe type, which includes basic applications such as Microsoft Word. Navidad is the Spanish word for Christmas.

McAfee's AVERT (anti-virus emergency response team) says more than 40 instances of the virus have been reported in the last three days.

This particular virus is proving troublesome because it doesn't contain a specific subject header that can help people identify it. Instead, it comes as a reply message that bears the recipient's own subject header.

If the infected attachment is run, a dialog box appears with an error message reading "UI." An icon of a blue eye then appears next to the clock icon in the system tray at the lower right corner of the PC screen. The Trojan horse virus is saved to the file "winsvrc.vxd" in the Windows system directory.

You can terminate the virus by clicking on the blue eye and closing the dialog box, using the small "X" that appears at the upper right hand corner of the box. The dialog box should display a large blue button labeled "don't press me." When another message box appears, you can click "OK" and terminate the program, making the eye disappear.