The document outlines several common effects that viruses can have on computers. These include filling the computer with useless files which takes up storage space, messing up files by making parts unlocatable, corrupting the file allocation table which tracks file locations, damaging the boot sector so the computer cannot start up, erasing the entire hard drive or diskette, directly resetting the computer, slowing down processing speed, corrupting files so they cannot be opened, redefining keyboard keys to trigger harmful actions, locking the keyboard entirely, and stealing private information for malicious purposes.