This one's mainly about Internet security. Suppose a malicious person writes a program designed to do nasty things to the host PC. The program could be given the same name as a legitimate Windows system file, or one that ships with a popular piece of software. This bogus program might be incorporated into some attractive item of software that many people will be tempted to download over the Internet.

You install the software which may in fact do what it is supposed to, so as not to arouse suspicion. Unknown to you, the installation process has also overwritten the original system file with the malicious version of the same name. When Windows or the well-known application executes the program in this file, thinking it will perform some perfectly innocent function, it actually triggers the destructive software.

Result: whatever dark purpose the programming terrorist had in mind.

Similar problems might arise with ActiveX controls on Web pages, or a bogus device driver or e-mail... forgery has the potential to become a major menace on the Internet.

Signature Verification Tool is part of a strategy that aims to stop all that. It should enable you to verify the originator of a file and that the file has not been tampered with or damaged. It works like this. A recognised authority certifies program authors each of whom has two codes: a public key and a private key. The public key is widely known, whereas the private key is kept secret.

A program (or other file) which is to be signed is first run through a utility which calculates a value based on the contents. This and the author's certificate are encrypted using the private key and inserted into the file. Signature Verification Tool and other security programs can then use the public key to read the encrypted information. The public key cannot be used to encode the data and thus mimic the author's signature.

Only if the file contains a valid certificate and the contents match up with the calculated check value is the file considered authentic and in its original condition.

Signature Verification Tool will search your disk for signed or unsigned files – choose which in the Look For box at the top of the window. If you want to, select a more precise location than everything in My Computer, and/or files whose names match a pattern. Click Find Now to start the search. It takes a while for unsigned files, and much longer when looking at signed files. You end up with a display similar to this:

You can click once on any signed file and then on Details, and the certification information will be displayed in the tabbed dialog shown in the following three shots:

What happens if a signed file has been altered? It simply doesn't appear in the list of signed files, but instead in the list of non-signed files. So you've got to know what you're looking for with Signature Verification Tool. It won't sound alarm bells if there's a mismatch between content and signature. Use it to verify files whose expected signature status you already know.