Malicious software code can be hidden an e-mail's time and date stamp through a "buffer overflow" - superfluous letters and numbers that trigger an error. Following the spurious characters, a hacker can include software code that is recognised by the computer as legitimate instructions.

Russ Cooper a security expert said "from there, I could do anything that I would normally be able to do on my computer." So effectively the hacker now "owns" your machine.

Microsoft stated that the problem component is actually in Internet Explorer, and suggests users upgrade to Internet Explorer version 5.01 Service Pack 1, which can be found free on Microsoft's Web site. That version is not vulnerable to this problem. Internet Explorer 5.5 is also safe for all users except for people running the Windows 2000 operating system. Those users should also get IE 5.01 SP1.

The buffer overflow trick is not just restricted to e-mails. There are reports that form fields on certain Web sites have exhibited similar characteristics.