The intruder installs a rootkit on a computer using a user action or by exploiting a known vulnerability or cracking a password. The rootkit installs a backdoor giving the hacker a full control of the computer. It hides their files, registry keys, and process names, and network connections from your eyes.
Your antivirus could not detect such programs because they use compression and encryption of its files.
UnHackMe allows you to detect and remove Rootkits.
This software is a shareware. You will be able to download and test UnHackMe during a certain period of time, then, if it does what you need, you will have to acquire the full version from UnHackMe publisher. The UnHackMe 4.0 free trial version contains an installer and an uninstaller, and has a size of 1079 Kilobytes.
For additional information and support request, please contact directly UnHackMe publisher.

UnHackMe Screenshot