Nirsoft specifically addresses this in their FAQ on the site.

Q: The antivirus software on my computer reports that some of the your utilities are infected with a virus or trojan. What should I do ?
A: First of all, All the utilities in my Web site are clean, and they don’t contain any virus or trojan. Unfortunately, Antivirus programs are not perfect, and in many times they detects innocent software as infected with trojan or virus. This problem is known as ‘False Positive’ or ‘False Alert’, and it’s quite common in password-recovery tools. If your antivirus software reports that utilities you downloaded from this Web site are infected with a virus or trojan, I highly recommend you to contact your antivirus company and ask them to fix this ‘False Positive’ problem in the next update of your antivirus software. You can also contact NirSoft to report about ‘False Positive’ problems. Click here to view the latest ‘False Positive’ problems reported by users from around the world.”

It’s a common problem for them as many users have reported the issue. The problem lies more with the anti-virus providers being tripped up by things like seeing the string “Password Recovery” or “crack” or “patch”.

I have downloaded and run this tool and can tell you it does not contain any viruses.

To run it, go to safe mode and install it from there.

Punchy