You probably have either:
a) a setting in BIOS that tells the system to "seek a bootable floppy" on startup - or something similar
or
b)
your virus-checker (if any) is set to scan the floppy during the load of the OS.
It's more likely that a) is the answer if it happens very soon after you switch the machine on.
To fix, edit the BIOS (the exact way to do that depends on your BIOS version/computer type etc) or re-configure the virus checker if required.
hth
amofw