You can't usually make up a clean boot disk on a system which has been booted from an infected floppy or hard disk. So how do you know you're booting clean? Actually, you can never be 100% sure. If you buy a PC with the system already installed, you can't be sure the supplier didn't format it with an infected disk. If you get a set of system disks, can you assume that Microsoft or the disk duplicator didn't somehow release a contaminated disk image? (Yes, something rather like this has indeed happened...) However, you can be better than 99% sure. * If you have (and use) a reputable, up-to-date virus scanner, it will almost invariably detect a known virus in memory (scanners can't be relied on to detect an unknown virus, in memory or not). If a good scanner doesn't ring an alarm bell, you've *almost* certainly booted clean. What constitutes a good scanner is another question, however. * If you have a set of original system disks which you received shrinkwrapped *and* which you've never used *or* which have only been used write-protected, you can probably use Disk 1 as a boot disk and it *probably* isn't infected - after all, Microsoft doesn't use MSAV for jobs like this..... It has been reported, though, that DOS systems disks have been distributed infected, and the fact that they're often distributed write-enabled doesn't inspire confidence. * You could always contact the supplier of your most-trusted anti-virus utility and ask whether you can send them a boot floppy to check. Of course, even anti-virus gurus sometimes make mistakes, but a boot disk verified in this way would still be worth paying for, especially for organizations with mission-critical systems.