Microsoft Scandisk and Defragmenter have been part of the windows package since well, a long time,  these tools as we know them now came to life with windows 3.x but there were similar disk tools from none Microsoft sources before this.

Today these two tools are just as useful as ever and even the most novice of users can take full advantage of them.

Disk defragmenter.
If you think of your hard disk as a big piece of paper which your computer writes information to time and time again as you do more things, save documents, delete documents download files, music, movies etc.

Overtime as you delete, save, add to these bits of information there will be times when on this "big sheet of paper" there is no room in the same area of the paper to continue writing the same file and so instead a note is made to continue reading this file further down the sheet. With this in mind it's easy to see how this data can quickly get a bit mixed up and hard to read.  It would be impossible for us as humans to do this reading a few words then being told to skip a few hundred words to read the next half paragraph and then again and again. A computer can do this but at the cost of time, too much fragmentation can slow a computer down and affect performance.

What disk defragmenter does is it looks at your hard disk and rearranges all the data files so as many as possible are continuous files and there are no gaps in the data. This means that the data can be read quicker and that future data written will be placed directly in a nice empty space.

Scandisk
Scandisk in windows 7 and after is shown in windows tools as "error-checking",  but by whichever of the names we use its the first tool most people should look to when things start going wrong. When windows starts to do weird things a simple hard disk scan can very often resolve the problem. common causes for the errors are programs that did not terminate correctly or failing to shut down windows properly.