THIS SOFTWARE IS FOR ADVANCED USERS ONLY. USE AT YOUR OWN RISK, SERIOUSLY. That's not a joke. This program can seriously mess up your PC, so don't use it, or if you do, make ABSOLUTELY sure that you know what you're doing. And most of all, don't come crying to me if you screw something up, because, hey, I warned ya.
Now that that's over, HijackThis is a removal tool that scans "your PC and generates a log file of registry and file settings commonly manipulated by malware as well as good software." The main feature of HJT is the system scan. Once clicked, it scans your registry and other files, and brings up a list. Every item has a code before it, like "R1", which corresponds to what it is. O2 is a Browser Helper Object, R0 is a changed registry value, and etc. If you're nto sure what something is, click "Info..." in the scan section to show a complete list, or if you want a more detailed explanation for one entry, click "Info on selected item...". However, this doesn't tell you want the actual entry is, just the type, so still be careful.
Anyway, you check what you're sure is bad (or unwanted), and click "Fix checked," which is another way of saying "Delete checked." That's about it for system scan. You can also save it to a log as well. Alot of times, if you ask someone online (or a friend) about a problem that your computer is having, they might say "Post your HJT log."
Now for Misc. Tools. HJT can also create a "StartupList log," which, despite its name, does more than startup. When you run it, it creates a TXT file with all running processes, startup folders, startup registry entries, and much, much more. HJT also has a very simple process monitor, a HOSTS file editor (very useful), "Delete a file on reboot," an uninstall list editor, and more.
HJT is also portable, since it is very effective to take from computer to computer, or you can install it. It's also tiny, at only 350kb, and standalone, at that.
I recommend using HJT on a computer at least once. It's not as important to do it a second time, that is, unless something goes wrong, like spyware or a virus. But beyond those regular threats, HJT can also help with removing McAfee or Norton, in case an install goes wrong. (There's also things like the Norton Removal Tool, but there's nothing for McAfee that I know of....) If you didn't know, have two anti-virus programs installed on one computer is very bad, and can make the comp almost unbootable sometimes. And sometimes (most of the time), McAfee's uninstaller doesn't get everything, which screws up the other AV. So use HJT to help get rid of the old, and then use the new. Just one of the many applications of this program.
So HJT is a very useful program, but you must make sure that you know what you're doing. There' warnings all over, on the site, and even in the program. (The first time you run it, it makes you check a waiver.) But if you can figure it out and use it, it's a lifesaver.