Adam, a novice computer user, works primarily from home as a medical professional.

Adam, a novice computer user, works primarily from home as a medical professional. He just bought a brand new Dual Core Pentium computer with over 3 GB of RAM. After about two months of working on his new computer, he notices that it is not running nearly as fast as it used to. Adam uses antivirus software, anti-spyware software, and keeps the computer up-to-date with Microsoft patches. After another month of working on the computer, Adam finds that his computer is even more noticeably slow. He also notices a window or two pop-up on his screen, but they quickly disappear. He has seen these windows show up, even when he has not been on the Internet. Adam notices that his computer only has about 10 GB of free space available. Since his hard drive is a 200 GB hard drive, Adam thinks this is very odd.

Which of the following is the mostly likely the cause of the problem?