Since 1993, I've been working, living and breathing technology. Computers have come and crashed throughout my graduate school, home offices, consulting and travel warrior days. The most reliable computers I have still in my possession are Apple and Toshiba. Some of my old computers date up to 10 years and are still in working condition. I've kept them to run old software that is severely outdated but still perhaps better than the stuff that is being created today.
My Powerbook G4 is still my main home computer. Although it is a good four years old, it has never failed or crashed on me. I also have never experienced a virus on an Apple computer.
Toshibas are built to last. I have an old Satellite running Windows95 and a Tecra running Windows98. Both are in perfect condition and the Satellite is a backup computer at a medical clinic here in South Florida. It has proven to be more reliable than their newer more advanced computer.
If your business demands buying computers in bulk, Dell is the most inexpensive option. I've used Dell laptops in most of my technology jobs. Midsize to large companies seem to love it.