1. "The growth of viruses is directly correlated to the interest of malware writers to develop malware for that OS." -Kaspersky
Only a minute percentage of users use Linux or Unix-based systems so, while virus-creators could create viruses for Linux if they wanted, they don't. Same reason kung bakit almost nonexistent ang viruses sa Mac OS. Not because it's immune against threats, but because konti ang users, and the virus-creators don't bother.
2. Depende sayo. Most Linux flavours are hard-installed, but if you really want to sample it muna, you can boot from a Live CD. Knoppix and Kubuntu come to mind. If you do a hard install, it can coexist with a Windows install through a boot-loader upon startup (GRUB or LILO). Yung run from Windows app naman ala Virtual PC, it's not native. Bale Linux Shell lang mangyayari sayo nun.
3. I've used Ubuntu for a thesis and it's OK. Hardly ground-breaking, but it's stable and it's the best when it comes to open-source writing. Most types of software have open-source equivalents for use in Linux. You have Gimp, OpenOffice, etc. If you need to play games, some have Linux installs like Doom 3 and Unreal 3. Still, there's Cedega and Wine which allow you to emulate PC games and/or applications.
IMO, Linux is very capable and very functional. If you choose a good kernel, it can even be more stable than a Windows OS - which is good if you plan on making a home-based server. However, if you rely on many Windows-only (or Mac-only) programs, then maybe it isn't for you.