rootkit

Rootkits are Trojans, that employ various active measures to avoid detection for itself and other malware. For example, rootkits might modify process list, file lists so their file and processes won’t be scanned and thus detected. Typically, this kind of computer parasites might block antiviruses too, cause browser redirection from antivirus websites and offers some sort of backdoor functionality. While rootkit itself will not be seen, the malware downloaded will produce some sort of symptoms usually.
It is quite tricky to remove rootkits even with automated software. In many cases alternate OS scanners should be used, although some anti-malware and specialized tools have good anti-rootkit capabilities.