How to prevent my disk usage from being 100% all the time?

I suspect 2 issues: not enough free space on your HD and/or too many programs in the startup group. Other possible problems are malware (this is different than virus) and not enough RAM.

I am assuming you are running an up to date anti-virus and are also doing Windows Updates.

In Win 7, press Win key-E and look at the thermometer gauge under the C drive. At least 10% and preferably 20% of your HD should be free space. If it is under 10% your computer will slow to a crawl. You may have to move videos, pictures and other large files to another drive to gain space. Uninstall programs you seldom use. In all OS's press Win key-E, right click on the C drive on the left side of this window, and select Properties for more detailed info.

Run CCleaner and Malwarebytes to get rid of malware. The CCleaner, Tools, Startup will allow you to see what programs are running every time you run your computer. Disable those that do not need to run all the time. Google the names of programs you are unfamiliar with to see their function and if they are safe to disable. Disabling does not uninstall. It simply stops these programs from running every time you boot the computer. You will still be able to run these programs any time you want.

If IE or Firefox is very slow opening pages, go to Help and select "Start without add-ons" and see if this speeds things up. If it does, go to Tools, Manage Add-ons and disable add-ons until things speedup. Uninstall browser toolbars and search engines that you do not use. Simply not using them is not enough.

Games may take up a lot of room. I found an excellent free program to tell me where my hard drive space is being used.

http://www.jam-software.com/treesize_free/

Files that you don't use all the time can be stored on an external drive. 3 TB drives go for about $120.

Well

Ccleaner is the application I use to clean up my system when my pc slows down because of viruses, trojans or malwares. Free Download here: http://bitly.com/UrAS6C

Very useful program!

Cheers.

Delete the larger files, videos, music, images etc. But if this keeps happening, you need a bigger drive.