The trojan, the malware analysis specialist said, is present on websites hosting illegal applications such as cracks, key generators and serial numbers for popular commercial software applications.
The trojan is typically used to force advertisements inside the browser, BitDefender said.
Ranking second, was Trojan.AutorunInf.Gen, a generic mechanism used to spread malware via removable devices such as flash drives, memory cards or external hard-disk drives.
According to BitDefender, Win32.Worm.Downadup and Win32.TDSS are two of the most famous families of malware to use this approach to trigger new infections.
Third slot in the October chart was taken by Win32.Worm.Downadup - also known as Conficker or Kido, the worm which restricts access to websites associated with IT security vendors.
More than that, BitDefender said, the latest variant of the worm installs rogue security software on the compromised machines.
And in fourth position came Trojan.Wimad, which takes advantage of a less-known feature implemented by Microsoft in order to store co-ordinated digital media data.
The trojan affects ASF files, an extensible file format that supports data delivery over a wide variety of networks and is extremely easy to play back locally.
A specially crafted ASF file abuses the feature which allows it to download the appropriate codec in order to install trojans instead.
In the fifth slot came Exploit.PDF-JS.Gen, a generic detection for specially crafted PDF files that exploit different vulnerabilities found in Adobe PDF Reader's Javascript engine, in order to execute malware on the user's computer.
Upon opening an infected PDF file, a specially crafted Javascript code triggers the download of malware binaries from remote locations.
Win32.Sality.OG took the sixth slot. This is a polymorphic file infector that appends its encrypted code to executable files (.exe and it deploys a rootkit and attempts to kill anti-virus applications installed locally.
Seventh place went to Trojan.Autorun.AET, a trojan spreading via the Windows shared folders, as well as through removable storage devices. The trojan exploits the autorun feature implemented in Windows for automatically launching applications when an infected storage device is plugged in.
Worm.Autorun.VHG is an internet / network worm that exploits the Windows MS08-067 vulnerability in order to execute itself remotely using a specially crafted RPC (remote procedure call) package (an approach also used by Win32.Worm.Downadup). The worm ranked eighth this month.
In ninth position, Trojan.Swizzor.6 is yet another variant of the Swizzor family. The trojan adds its key to the Windows Registry in order to execute a copy of itself each time Windows is started.
And finally, ranking last in this month's top 10 e-threats came Gen:Adware.Heur.wq0@j4oukhe, a generic routine that detects a wide range of adware applications, especially the NaviPromo family.