If your PC is infected with boot-sector virus. Scan with a good antivirus like Norton.
If indeed your MBR has been changed, Norton will display will big dialog saying -
'Your Boot sector which contains critical information has changed' There will be
options available like 'Update the MBR', 'Restore MBR' Unless you have made changes
in Windows that affect the MBR. You must restore the MBR to it's previous state.
Still the virus is there!!
Use Seagate's DiskWizard Starter Edition. This program does a low-level
formatting & destroys MBR & the boot-sector virus!! Caution: All your data will be
lost!!
For instructions on how to use DWSE, see here
Download DWSE from here
You must not download it in your PC, since it's already infected with the virus!!
You must download it in a clean virus-free PC , choose 'Bootable CD ISO Image'.
Use Nero, & choose 'Burn as an Image' & select this DWSE's ISO file.
Now you must boot from this CD in your virus-infected PC. Choose 'Utilities'
'Quick Zero Fill'. That's all in 3 secs MBR is over-written & the virus is removed
from MBR. Now install Windows. Your PC must be virus-free now.