John Guarino is the owner of TecAngels, a two-man computer consultancy in Manhattan. Give Guarino your ailing Windows PC, and in two or three hours he'll return it to you in perfect health. Often, he can solve his customers' problems over the phone.

But last summer, Guarino came across a problem he couldn't fix. In the process of flushing out the spyware and viruses infecting his customers' computers, he began to find the same mysterious intruders in machine after machine. They were strangely named files lurking deep inside the "registry" where Windows stores settings and instructions that control all of a computer's hardware and software.