First, what is a disk scan? Well, a disk scan checks your disk to see if there are any errors that need to be fixed. It checks each file on your disk, indexes (which basically make it easier for the computer to find files) and sectors (which are specific locations where data are stored) to see if any of these need to be addressed. If it finds any issues or errors, most of the time, the disk scan can take care of them.
Now, why do we need to a disk scan? Just like any other piece of metal, the disk can begin to deteriorate over a period of time, especially with how much action it gets just by turning on your computer. You also have dust that collects inside your computer, which itself can cause some issues. Sometimes part of the disk degrades and needs to be fixed.
Once the disk scan has finished, you've only done part of your maintenance. The other part of disk maintenance is defragmenting the disk. What is defragmenting, you ask? Well, let's first cover what a computer does when you create a file.
Let's say you create a Word Document and you are ready to save it to your hard drive. When you actually save the file, the computer attempts to store the file on your disk, but because of the way computers work, the file can't always be saved in one place on the hard drive (because of size limitations, which is a new discussion in and of itself). So you may see one file spread across multiple sections of your disk. This spreading of files is called fragmentation and can cause a slow down in your system, if left alone long enough. So, defragmenting the disk does just the opposite: it takes the files on your disk and puts them in order and together as best it can. Thus, you can speed your computer back up.
Disk scan and defragmenting your disk are pretty simple tasks to perform. If you open Windows Explorer (go to your Start menu | All Programs | Accessories | Windows Explorer). From there, you should be able to find your C drive, which you should right click and select Properties. You will then see a Tools tab. The first option is "Error Checking", which is your disk scan. Click on "Check Now", and check the two options you see ("Automatically fix file system and errors" and "scan for and attempt recovery of bad sectors"). Then you will be asked if you want to schedule this scan for your next restart (since the disk can't be checked while you are using it). Click ok and then restart your machine. Windows will take care of the rest.
For the defraging of the drive, you will want to go to the same Tools tab, and under Defragmentation, click on Defragment now. In any operating system earlier than Windows Vista, you will click on Defragment in the new window, while with Vista and Windows 7, you will click on Defragment now.
Performing these simple tasks regularly (disk scan once a month and defragment once a week) can greatly increase the performance of your computer and avoid issues in the future.