MSAV Detects a Boot-Sector Virus but Fails to Clean It (96557)

The information in this article applies to:
    Microsoft MS-DOS operating system 6.0
    Microsoft MS-DOS operating system 6.2
    Microsoft MS-DOS operating system 6.21
    Microsoft MS-DOS operating system 6.22

This article was previously published under Q96557


Microsoft Anti-Virus may detect a boot-sector virus (such as STONED or Michelangelo) but fail to clean it. This problem can be caused by either of the following conditions:
    The boot-sector virus is on your DoubleSpace-compressed drive and your host drive.
    All the virus code is not being overwritten.


The Boot-Sector Virus Is on Your DoubleSpace-Compressed

Drive and Your Host Drive

Boot-sector viruses "infect" a drive by replacing the startup (boot) code that is stored at the beginning of the drive. A boot-sector virus is activated when you start your computer from the infected drive (usually drive C). Because the startup code on a DoubleSpace-compressed drive is never run, it is not possible for a boot-sector virus on a compressed drive to become activated. However, if a boot-sector virus is detected on a compressed drive, it is likely that the host drive is also infected.

You can detect and clean viruses from all local drives by using the following command:
   msav /c /l
The /C switch causes Microsoft Anti-Virus to scan and clean the disk. The /L switch causes Microsoft Anti-Virus to scan all local drives except A and B.

All Virus Code Is Not Being Overwritten

When a boot-sector virus infects a hard disk, it moves the original boot- sector information to another location on the disk and replaces it with virus code. Microsoft Anti-Virus finds the original boot-sector information and uses it to overwrite the virus code. If the original boot-sector information is incomplete or has been damaged in some way, some of the virus code may not be overwritten. When Microsoft Anti-Virus scans the drive and finds the remnant of the virus code, it reports it as a virus.

WORKAROUND If CHKDSK Reports 655,360 Total Bytes Memory

If the Total Bytes Memory reported by MS-DOS CHKDSK is 655,360 bytes, the virus is not active, and the following steps should be taken to clean the remnant of the virus code from the drive:

    Back up all data on all partitions on the hard disk drive. You may want to use the Microsoft Backup Compare feature to ensure that all files have been backed up successfully.
    If the infected hard disk drive was partitioned with MS-DOS Fdisk, the virus remnant can be cleaned by running MS-DOS Setup with the /M parameter. This refreshes the master boot record (MBR) and overwrites any remaining virus code.

    To prevent the virus from infecting your MS-DOS disks, ensure they are write protected before running MS-DOS Setup. To prevent your Uninstall disk from becoming infected with the virus, turn off your computer and then start it from a floppy disk known to be free of viruses before running MS-DOS Setup.
    Run Microsoft Anti-Virus to determine whether the procedure eliminated the remaining virus code.

WORKAROUND If CHKDSK Reports Less Than 655,360 Total Bytes Memory

If CHKDSK reports less than 655,360 total bytes memory, the virus may still be active. In this case, use the following steps:

    Boot from a write-protected system disk that is known to be free of virus infection (such as your original MS-DOS 6 Upgrade Setup Disk 1).

    NOTE: Although the 1.2-megabyte (MB) 5.25-inch floppy disks are write protected, the 1.44 MB 3.5-inch floppy disks are not. If you have the 1.44-MB floppy disks, slide the write-protect tab so that the write- protect slot is open on all your disks. Scan the floppy disks to ensure they are not infected. If they are, order new disks from Microsoft Sales Information Center (MSIC) at (800) 426-9400.
    Insert Disk 3 of either the 1.2-MB or 1.44-MB disk set in drive A.
    Change to the MS-DOS command prompt at drive A and run Microsoft Anti-Virus. For example, type the following at the MS-DOS command prompt, pressing ENTER after each line:

    msav c: /c /l

    Remove the floppy disk from drive A and restart your computer.
    Scan all hard disk drives to ensure the virus has been removed.
NOTE: The new location of the original boot sector information may be different for each boot sector virus. Microsoft Anti-Virus can therefore clean only the boot sector viruses on the Microsoft Anti-Virus virus list.

Which Boot Sector Is Executed During Startup?

If drive C is uncompressed, its boot sector is run during system startup.
If drive C is compressed, the boot sector of its host drive is run.

What the Boot Sector Does

The boot-sector startup code in MS-DOS versions 5.0 and later does the following:
    Confirms that the system files (IO.SYS and MSDOS.SYS) are the first two files in the root directory of the drive.
    Loads and executes the first three sectors of the IO.SYS file.
The boot sector is executed by the master boot record (MBR). If you have more than one partition, the MBR determines which one to run using the partition table.

How Boot-Sector Viruses Get on a Compressed Volume File (CVF)

If a drive is infected before you install DoubleSpace, some boot-virus information may be replicated on the DoubleSpace CVF. Although it cannot affect the system from the DoubleSpace volume, the signatures may still be detected here. To correct this, clean both the host drive and DoubleSpace- compressed drive.

Modification Type: Major Last Reviewed: 11/20/1999
Keywords: KB96557