How do you fix a master boot record?

You can repair a computer that has experienced this problem by booting from an MS-DOS boot disk or from a Microsoft Windows 95 or Microsoft Windows 98 Startup disk and then running the fdisk /mbr command. After you run this command, reboot the computer to cause the partitions to be re-created correctly.

However, your question didn't say the disk type, dymanic or basic? If the above fails see KB320283.

Before performing a fdisk/mbr, Check your BIOS boot sequence settings to make sure that the boot drive is listed in the boot sequence. For whatever reason, that may have changed. Check your physical connections e.g. data cabling, power cables and so on. Try the drive on another known working computer. Does the problem follow the drive?

To replace a Master Boot Record on a FAT FAT32 formated primary DOS partition, from a W9X boot disk at the A: prompt type chdir c:\ at the C: prompt type fdisk/mbr this replaces the current master boot record with the W9X default master boot record.

The first thing to do when you think that you mbr is corrupted because of a virus, if I'd be you I'd try to have a bootable floppy disk with an antivirus on it and try to run it. Then, if this fails, I'd get the HDD to another computer (as slave) and try to clean it using a specialised antivirus software. If none of the above works, I'd try to create a new mbr record.
You don't say anything about your computer system - so I assume that you are talking about W2K PRO or XP. So, you use your W2K or XP CD, you boot from it, you choose repair when asked and when you enter in the "dos-like" console you type: fixmbr command. To have a list of all commands available in this "dos-like" environment you can type: help.

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