What is it?
Google Toolbar - googletoolbar2.dll

What does it do?
googletoolbar2.dll - If you're an Internet Explorer user you will want to have this toolbar addon for your browser. It blocks some popups (more and more are getting around it) along with adding that spiffy google search bar for easy google searching. (That is one of the cooler parts about Firefox)

This is described as a BHO or Browser Help Object. If you're looking at your HJT log you will see a line that looks something like this:
O2 - BHO: Google Toolbar Helper - {AA58ED58-01DD-4d91-8333-CF10577473F7} - c:program filesgooglegoogletoolbar2.dll

Virus Precautions:
You'll want to keep an eye on this google search for any known viruses. The normal location of this file is C:program filesgooglegoogletoolbar2.dll