[dogscoff]

A lot of viruses are designed to slow down your Internet connection... don't know WHY that's all they do (as opposed to formatting your hard drive) if they got into your computer to begin with, but they do...

It's not so much them trying to slow down *your* internet connection as to perform a Denial of Service on someone else (ie Microsoft) which basically amounts to slowing down that company's connection by using yours and those of many other infected machines simultaneously to overload it.