What is mostly likely the cause of Travis' problems?

Travis works primarily from home as a medical transcriptionist.
He just bought a brand new Dual Core Pentium computer with over 3 GB of RAM. He uses voice recognition software to help him transfer what he dictates to electronic documents. The voice recognition software is processor intensive, which is why he bought the new computer. Travis frequently has to get on the Internet to do research on what he is working on. After about two months of working on his new computer, he notices that it is not running nearly as fast as it used to. Travis uses antivirus software, anti-spyware software, and always keeps the computer up-to-date with Microsoft patches.
After another month of working on the computer, Travis’ computer is even more noticeably slow. Every once in awhile, Travis 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. Travis is really worried about his computer because he spent a lot of money on it, and he depends on it to work. Travis scans his computer with all kinds of software, and cannot find anything out of the ordinary. Travis decides to go through Windows Explorer and check out the file system, folder by folder, to see if there is anything he can find. He spends over four hours pouring over the files and folders and cannot find anything.But, before he gives up, he notices that his computer only has about 10 GB of free space available.Since his hard drive is a 200 GB hard drive, Travis thinks this is very odd.
Travis downloads Space Monger and adds up the sizes for all the folders and files on his computer. According to his calculations, he should have around 150 GB of free space. What is mostly likely the cause of Travis’ problems?

A.
Travis’s computer is infected with Self-Replication Worm that fills the hard disk space

B.
Logic Bomb is triggered at random times creating hidden data consuming junk files

C.
Travis’s computer is infected with stealth kernel level rootkit

D.
Travis’s computer is infected with Stealth Trojan Virus

Explanation:
A rootkit can take full control of a system. A rootkit’s only purpose is to hide files, network connections, memory addresses, or registry entries from other programs used by system administrators to detect intended or unintended special privilege accesses to the computer resources.