"HJT" stands for "HiJack This". It is a program that analysis what is installed and running on your machine, and then allows you to "fix" things that you do not want on your computer. It is often used by people whose machines are not acting properly due to unwanted malware they have acquired.
Many people run HJ-T to get a log of what is on their machine, and then post that log in the Security forum for an expert to interpret. You may want to browse the Security forum to see what the logs look like, and how people give advice based upon the logs.
HJ-T is free, get it here:
http://mjc1.com/mirror/hjt/