Invasion into the bootsector

How do I get rid of 3 offensive pictures, which have surreptitiously insinuated themselves into my Windows 98 boot sector? Antivirus scans, including MS-DOS scans, come up negative. These pictures are visible just before the red “COMPAQ” letters upon booting, and they replace the “NOW IT IS SAFE TO TURN OFF THE COMPUTER”, just before the shut-off. When turning off all items in the Selective Startup of the System Configuration Utility and rebooting, the pictures remain in place.