In it's own thread due to it's excellence and wide recognition by malware-busters everywhere. This is an extremely widely-used, accepted tool for assisting in removing stubborn malware.
Here is the official homepage / download location for Merijn.org's "Hijack This" tool:
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http://www.merijn.org/programs.php#hijackthis <<<
It's not an actual antivirus or spyware remover program itself, as-such.
What it does do is read all manner of highly technical info about malware out of your infected computer, and place it in a simple text file.
The idea is, you then give that log file to someone you trust on the web (usually in a dedicated malware removal forum, by copy/pasting) - and they analyse it for you, and tell you step-by-step precisely what to do, in order, to remove the malware.
You don't have to understand the contents of the logfile, or even bother to read it. Just follow the directions given by the person analysing it.
Of course you need to satisfy yourself that the forum in which you are asking for help is a well established, widely trusted group - ask around on the net, use Google. Read existing posts on the board & see what sort of outcomes are there before diving in; do people generally come back with a thanks.. or do they simply disappear mid thread all the time..
For each malware listed we may link to the occasional removal forum for reference - although all info is posted in good faith in the hope that it may prove useful, the responsibility for making the final decision as to whether a malware removal forum is 'legit' and provides sound advice, rests with you.
You should download a new copy of Hijack This every time you need to use it, as it is frequently updated.
If you want to analyse your logfile & repair your computer yourself, (do so entirely at your own risk!), some help is here:
http://www.merijn.org/htlogtutorial.php