Systems administration unfortunately requires a lot of defensive work against viruses. A huge range of tools are ranged against ever more sophisticated enemies.

The basic groupings of files that might cause problems are: Boot Viruses, Worms, Macros and Back Door/Trojans

Worm
Vulnerabilities in an application or system or attacked by this type of virus. Infected computers are in turn used to find other machines to infect. Thus propogating the virus.

Macro
Microsoft applications seem particularly hard hit by Macro viruses. I do not know if this a) hackers like attacking Microsoft's punters, b) they are easy to attack or c) this is the quickest way to get to millions of PCs. My guess would be c). Macros work by by abusing the scripting functionality of Office type applications. Word processing, spreadsheet and database applications are all affected.

Boot
The grand-dad of all viruses is not seen often these days. The virus affects either Master Boot Record (MBR) of a hard disk or the boot sector of a floppy disk.

Back Door/Trojans
Potentially the most dangerous of attacks. The back door or trojan attack is not even strictly a virus as they do not replicate. Their danger lies in the ability to reside in the background and be control the system remotely. Even to the extent of attacking other systems.