Trojan:Win32/sirefef.O removal issue is a typical problem irresolvable for antivirus tools that do not properly analyze all types of files susceptible to the infection. That is why infected files keep infecting other files detectable for incomprehensive antivirus. Naturally, the antivirus detects and disinfects files that it can examine.
Payload of the infection is not permanent and might be altered up to several times a day. Of course, there is no reason to expect it to be safe for infected PC.  Start free Trojan:Win32/sirefef.O   manual removal process and get rid ofthis dangerous malware.