HijackThis, sometimes abbreviated HJT, is freeware spyware-removal tool for Microsoft Windows originally created by Merijn Bellekom, and later sold to Trend Micro. The program is notable for taking a heuristic approach on detecting malware - instead of relying on a database of known spyware it quickly scans a user's computer, creates a list of differences from a known spyware-free environment and allows the user to decide what from the list needs to be removed.

Use

HijackThis can generate a plain text logfile detailing all entries it finds, and most entries can be removed or disabled by HijackThis. Caution should be exercised when using the latter option, as HijackThis does not discriminate between legitimate and unwanted items with the exception of a small whitelist of legitimate entries - thereforeallowing a user to unintentionally disable important programs from running, which may possibly cause their system or peripherals to stop working. HijackThis will, however, attempt to create backups of the files and registry entries it removes, that can be used to restore the system in the event of a mistake.

A common tradition is to post the logfile to some web board/forum where more experienced users will help decipher which entries need to be removed. Automated tools also exist that analyze saved logs and attempt to give recommendations to the user, or to clean entries automatically. Use of such tools, however, is typically discouraged by those who specialize in manually dealing with HijackThis logs; they consider the tools to be potentially dangerous to inexperienced users, and neither accurate nor reliable enough to substitute for consulting with a trained human analyzer.

Later versions of HijackThis include extra tools like a task manager, hosts file editor and alternate data stream scanner.