- Connects to the internet without permission
- Installs itself without permissions
- Slow Computer
- Slow internet connection
- System crashes
Win32/Sirefef is a family of Trojans that have a big number of components each of which is responsible for a different part of the infection. Trojan:Win32/Sirefef.O is one of these components, and it allows rootkit components for this Trojan family to function properly. Unfortunately, properly functioning rootkits are not something a user would want on his computer, so Trojan:Win32/Sirefef.O has to be removed as soon as possible, but that might not be that easy to accomplish.
Since Trojan:Win32/Sirefef.O is a part of a big Trojan family, it is usually installed by one of the Trojan droppers from TrojanDropper:Win32/Sirefef group. Also, it is very hard to detect or notice the infection itself, because Trojan:Win32/Sirefef.O is no bigger than 1Kb in size. Nevertheless, it has a process file associated with it directly, and if that file is present among the currently running system processes in Task Manager, then you definitely have Trojan:Win32/Sirefef.O lurking in the depths of your system. The file name is prueba.exe, and be sure to check whether it is running, because looking for associated files is one of the best ways to detect Trojans that do not have obvious interfaces.
Even though Trojan:Win32/Sirefef.O does not influence any system file on its own, it is a constituent part of main rootkits from Sirefef Trojan family, and it is a direct indication that you are also infected with Virus:Win32/Sirefef.M. It functions as browser hijacker that modifies the user's search results, and also downloads additional components onto the target system.
The bottom line is that if you decide to remove Trojan:Win32/Sirefef.O (which you should), you will also have to remove all the other pieces of the big puzzle that is Win32/Sirefef. Since manual removal can turn messy, it you should avoid being swarmed by unsolvable tasks by choosing automatic removal. Get yourself a powerful security tool that will terminate Trojan:Win32/Sirefef.O and all the other Trojans from this family. Allow yourself to breathe a sigh of relief once the program deletes all the malware present.