Hacktool.win32.vb.gz

While I was using my computer and browsing online, an alert from my firewall showed that there was a HackTool.Win32.vb.gz virus. I looked for the path/location where the infected file was located but there was no such file. I have scanned my pc using AVG and Spybot SD but they were not able to detect the said virus. What is this? What should I do?