(1) With SAMBA - http://www.samba.org
(2) It is probably a software modem. Software modems are more of a telephone line interface then an actual modem, most of the job that should be taken care of by hardware is shunted off in to software. As this software is propriatry (and hardware modems are only about £10 more expensive then software modems) very few are supported under Linux. You could try http://www.linmodems.org/
(3) It depends on the file system. Most distros default to using the ext2 file system, which has a file size limit of 2GB.
(L) Viruses do exist on Linux, but only about a dozen, and they have enormous difficulty spreading themselves.