boot insto dos from a floppy and from command line, 'fdisk /MBR'.
if you dont have a bootable floppy,get windows install cd.
boot from it to Rescue mode by pressing R(or F5,i think) button.from there,try 'fixmbr <device_name>'.
device_name:The device (drive) on which you want to write a new master boot record. The name can be obtained from the output of the map command. An example of a device name is: