I worked on computers for the Army for the last few years before dropping my retirement papers. The military replaces computers on a 3 year cycle (in theory) and has used several brands over the last decade. The ones that have held up the best are Dells (I own one) and Toshibas (I would have bought one, but the Dell was cheaper). Dells seem to be fairly hardy (the cases DO scratch easily, but that's cosmetic, not functional), and are easy to repair. Replacing the hard drive on a Dell or Toshiba is 2 screws and a clip, where with a Mac laptop it is a miserable job, and a hard drive is the most likely point of failure on a laptop.

I would definitely get a carrying case for the laptop, especially if you are worried about dropping it. There are soft cases on the market which will put up with almost any abuse, even airport luggage handlers.

A computer getting a virus has nothing to do with the hardware, it is almost 100% a software issue. A virus can get into a system many ways, and an A/V program is just ONE piece of the defense against a virus. Macro viruses are easy to write and difficult to defeat, and normally are handled by MS system and software updates (Office updates, etc). Spyware and Malware which are not normally stopped by A/V programs can also cripple a computer.