I am not aware of an antiivirus boot disk. A floppy by itself could not possibly hold all the virus definitions. Even the most common ones.
If you can get that PC online I would go here
http://housecall.antivirus.com/housecall/start_pcc.asp
You may also want to check this site out
http://www.freebyte.com/antivirus/#scanners
I usually just remove the hard drive and connect it as a slave to a PC that as updated definitions and scan/repair it from there.
Good Luck.
J