Symantec Antivirus in my book. You have to pay per year
for that though. More info at www.symantec.com
A free antivirus can be found at www.free-av.com and it
seems to work pretty well.
I would use Ad Aware and/or Spybot Search and Destroy also
to make sure it's not spyware or a homepage hijacker or
something of that sort. Both of those can be found at www.downloads.com
