Presumo tu abbia cercato di seguire un procedimento di questo tipo (quello che riporto, in inglese, è quello consigliato dalla Sophos):
<BLOCKQUOTE id=quote><font size=1 face="Sans Serif, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote><b>Disinfecting master boot record viruses in MS DOS</b>
The boot sector must be replaced using the DOS utility FDISK.EXE:
* Switch off the PC, wait several seconds then boot from a clean system disk to prevent the virus from being loaded into memory.
* At the command prompt type
DIR C:
* Check that the contents of the infected drive are visible. If they are not contact support for advice.
* To overwrite the master boot sector enter
FDISK /MBR
If a virus fragment is reported in upper memory then it probably indicates the presence of a virus.<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Sans Serif, Arial, Helvetica" size=2 id=quote>
Ti do però anche un link ad una pagina di Symantec, che spero possa darti qualche buona idea:
http://www.symantec.com/security_response/writeup.jsp?docid=2000-122009-5001-99&tabid=3