It's possible that there is some form of infection, but it's hard to know for sure. With spyware and bots constantly evolving, users can become infected with a rare strain that antivirus and antispyware tools don't pick up, or that aren't evil enough to justify a signature. While antivirus and antispyware tools are certainly helpful in protecting machines, they focus on detecting widespread malware that is truly malicious. They don't do well with targeted attacks that are based on custom malware, or malware specimens that aren't widespread. Also, what one person considers resource-hogging spyware, another person may consider an interesting and useful application. Thus, it's not always clear which specimens the antivirus and antispyware tools should detect. That's why it can be immensely helpful to get a second opinion, just as one would for a serious medical condition.
To get such an opinion, I recommend running a scan using a free antivirus tool, such as Clam AV, and a free antispyware tool such as Lavasoft's Ad-aware. Run a scan with these tools to see if they pick up something that existing antivirus tool don't. Users also may want to run a scan using Microsoft's free malicious software removal tool. Although it's not as comprehensive as most antivirus and antispyware tools, it's excellent at removing some of the most pernicious malware specimens out there today, and it's updated fairly regularly.
This was first published in June 2008