This vulnerability, related to the CALL function of Excel, allows an attacker to send an HTML email or modify a HTML web page so that when accessed, the HTML page will automatically launch Excel and use that to run any program. This allows the attacker to do pretty much anything he wants on the host machine.
This attack is not widespread and no real-world incidents have been reported.
The problem has been partially solved by Microsoft by releasing patch for Excel 97 (this patch will disable the CALL command completely). Excel 95 can't be protected by this time.
Netscape and Internet Explorer can also be secured against this attack by updating to the latest version with latest patches. This does not prevent some attacks through HTML email though.
F-Secure Anti-Virus detects Excel files with embedded harmful CALL commands and will protect the user against this type of attack.
[By Mikko Hypponen, F-Secure]