This is a typical stealth boot sector virus, which beeps and displays this text after every 15 infected floppies:
ystanbulCCC was here! He he
Otherwise the virus just spreads.
IstanbulCCC was reported to be in the wild in the UK in April 1996.
Allow F-Secure Anti-Virus to disinfect the relevant files.
For more general information on disinfection, please see Removal Instructions.
Technical Details: Peter Szor, F-Secure, 1996