Why Windows blocks the installation of some files
To help protect your computer from potentially harmful files, Windows XP Service Pack 2 (SP2) will sometimes block the installation of files that you might request or receive over the Internet. This will happen if the file contains an ActiveX control or Web browser add-on that does not have a valid digital signature.

What are ActiveX controls and Web browser add-ons?

ActiveX controls and Web browser add-ons are small programs that are used extensively on the Internet. They can make browsing more enjoyable by providing toolbars, stock tickers, video, animated content, and more. These programs can, however, malfunction or give you content you don't want. In some cases, these programs can be used to collect information from your computer in ways you might not approve of, possibly damage data on your computer, install software on your computer without your consent, or allow someone else to control your computer remotely. Given these risks, you should only install ActiveX controls or add-ons if you completely trust the publisher. 

What is a digital signature and what does it tell me about a file?

A digital signature is a way to electronically mark a file with information. In this way, the file is signed by the creator (publisher). A valid digital signature tells you two things about a file: the publisher name and that the file has not been changed after it was signed. Any tampering makes the signature invalid.

If a file signature is invalid, or information in the digital signature seems suspicious, don't open the file. This helps you avoid situations where someone might post a file that does not belong on a Web site, or add harmful content to a file that isn't theirs.

Keep in mind that a digital signature does not promise a harmless file. A signed file could still contain harmful code if it was created that way. That's why, when you start to open certain file types from e-mail, Windows will ask if you want to open the file even if it has a valid digital signature.

Click Related Topics for information about digital signatures and how they can help you decide which files to open or install.

 Important

The default security settings in Internet Explorer are designed to help protect your computer from viruses and attacks over the Internet. Microsoft strongly recommends that you leave these security settings at their default level or higher.

 Note

Windows will also block files from publishers that you (or an administrator on your computer or network) have blocked. Click Related Topics for more information about blocked publishers.