You can protect your computer from accidental infection by setting preferences in Word, Excel and PowerPoint. With the proper setting, these programs warn you when you open a file that contains macros. Once you're warned, you can decide whether to continue opening the file, or whether to open the file with macros disabled, or whether to abort opening the file. Since macro viruses are macros, these options enable you to screen out unexpected macros, which are the chief source of macro viruses.

To set this preference in Word, Excel or PowerPoint in Office 2000:

Choose the menu command Tools>Macro>Security. The Security dialog appears. Click the Security Level tab to expose the Security Level options page. Make sure that the "Medium" radio button is on, and click OK.

To set this preference in Word, Excel or PowerPoint 2003:

Choose the menu command Tools>Options…. The Options dialog appears. Click the Security tab to expose the Security options page. Click the "Macro Security" button, and in the Security dialog box that appears, make sure that the "Medium" radio button is on, and click OK to close the dialog box. Then click OK again to close the Options dialog.

To set this preference in Word, Excel or PowerPoint in Office 2004:

Choose the menu command (Word, Excel or PowerPoint)>Preferences…. The Preferences dialog appears. Click the General tab to expose the General Preferences page. Make sure that the Macro Virus Protection check box is checked, and click OK.

To set this preference in Excel 2007:

Click the Microsoft Office button, then the Excel Options button, then Trust Center category, then the Trust Center Settings button, then the Macro Settings category. Click the radio button labeled "Disable all macros with notification." This is, by the way, the default setting, so unless you or someone else has changed the setting, you shouldn't have to do this.

Although you won't be submitting files to us containing macros (except as noted above), some of the files we deliver to you do contain macros. When Excel or Word warn you about these macros, you should accept the file and enable the macros.

Once you have set your preferences correctly, checking a workbook for macros is a simple task. If it's loaded into Excel already, close the file to remove it from Excel. Then load it again. If Excel warns you that the file has macros, then it has macros. If not, it does not. If you get a warning for a file that you do not expect to have macros, your computer could be infected with a virus. You should run your virus protection software to be sure.

If your virus protection software says that your computer is OK, and you still have macros in your workbook, create a new blank workbook, and save it with your name as its name. Email it to us and we'll examine it to see what's going on. Your computer might be infected.

Despite our best efforts and care, it's always possible that your computer might be infected by us or by some other source — or it might already be infected. To address this possibility, we strongly recommend that you acquire a virus protection software package, install it, scan your computer regularly, and keep your virus protection software up-to-date. Even better, configure the software to scan your computer automatically. This software is inexpensive, and a real must-have for any serious professional today. Three excellent products are available: VirusScan from ; Norton AntiVirus from Symantec, and Sophos Anti-Virus from Sophos.