The Trojan QAZ was first seen during the summer, according to anti-virus organisation Sophos.
It is a "backdoor Trojan that has worm characteristics", Sophos explained. The virus was ranked fifth in the company's list of top viruses for September.
When the Trojan horse is launched it searches for a copy of Notepad.exe and renames it to Note.com. It then copies itself to the computer as Notepad.exe.
Each time Notepad.exe is executed, the Trojan will run and then launch the untampered version of Note.com to avoid being noticed. It also allows remote hackers to connect and gain access to the affected computer when it is connected to the Internet.