LimeWire is the least vulnerable gnutella client because it's written in Java. This means it's essentially invulnerable to the worst kind of networked security problem, buffer overflows.

It is impossible for someone else to take control of your computer solely because you're running LimeWire.

Now -- it may happen that you download an executable that has a virus in it, but that's not because of LimeWire. It even warns you about attempting to download something that may contain a virus.