A part of a computer that keeps history even when erased in history menus?

My I.C.T teacher recently told us in a lesson that in a computer, there is one area that cannot be erased, and everything you do and access is there. He also said that the only way to clear this part is to completely empty and reset your computer, returning it to how it would be when bought. Is this true? If so, what is the name of this part? Can it be accessed easily? I'm going to have more virus protection updates installed by a family friend soon, and although I actually don't have anything to hide, I don't want him to be able to go through everything I've done when he feels a little bored and a quick rummage through my history takes his fancy. -.-

A part of a computer that keeps history even when erased in history menus?virus protection

It can be accessed and rewrote so all information is deleted. Any .dat files can store information about where and what you do. They can be erased and then rewrote but special programs are needed. The .dat files can not be erased or deleted after Windows has accessed them. To delete or erase the information they contain you need a program that creates a .bat file to run at startup before Windows has accessed them to clean them.

One program I know of is FREE and is called Index .dat Suite. Seems to be an OK program, it can allow you to visually see what sites a person has been to even after they THINK they have deleted all the history and cookies. I use it sometimes just to help make the .dat files smaller and seems like the computer runs faster to me, probably not and just my imagination, lol.