Looks like I am not the only one who has a virus at the moment. The trusty old Osborne ended up with what appeared to be an MBR or boot-sector infection. The Sound Blaster 16 installation disks I had been using were also detected as infected. At the time I assumed those disks had infected the PC. I cannot actually prove the direction of transmission though. Without a known-clean copy of the disks from before they entered the Osborne, the PC could just as easily have infected them.

I only discovered the problem after using the disks in the Osborne and then bringing them back to another machine to reformat and re-image. So a big thanks to Avast under Windows XP for sounding the alarm, and to Caluser2000 over at the Vintage Computer forums for pointing me towards a DOS-era cleanup tool.

F-PROT 3.16f saved the day. The scan report identified ABCD.A in the master boot sector. The archive I used was fprot_dos_316f.zip, with virus definitions dating into 2009. F-PROT 3.16f was the final DOS version and is perfectly at home on a 486.

For this sort of boot-sector cleanup, the important part is starting the infected computer from known-clean media rather than booting from the compromised hard disk. Ideally that rescue floppy should also be write-protected so the infected system cannot modify it.

Windows 95 is now installed as well, and the next challenge is getting this thing onto the internet to some degree.

Sources