Prior to boot sector viruses, it was rare that any support person even knew what a MBR was. For background on the MBR: MBR Explained. If your MBR has been contaminated by a virus, use the virus vendors document to recover it. If you can not, the preferred approach is to have a backup of the MBR. See Backup/Restore MBR. If its too late for that the next best approach is to rewrite the Master Boot Record using the DOS-based FDISK command:

fdisk /mbr

Use a dos boot disk and run it. If you don't understand what this means, don't try this tip.