My in-laws' computer is apparently infected with Packed.Win32.Tibs, and their antivirus can't disinfect...

They were using an outdated Symantec product, but I installed the Charter Communications High-Speed Security Suite (F-Secure). They had a recurring virus message before, and now after installing the new security suite, they are still getting a message that the antivirus program cannot disinfect "Packed. Win32. Tibs". What is the best way to get rid of this trojan? Thanks!