Not only does it slow the machines down, if often causes a lot of instability. I use Adaware (which has been greatly improved lately) to remove it.

Some infections, when removed, destroy your tcp/ip, so you have no network or Internet access. Just reinstalling Windows over the current install doesn't fix it. You have to go through the whole process of uninstalling tcp/ip, removing all it's files, and then reinstall Windows over the current install.

Attaching Windows to the Internet just gets more and more expensive by the month.