Did I catch a virus or something?

Question

Macbook Air Version 10.8.5
Ok, so on Tuesday I downloaded something and nothing really happened that day. Uninstalled the item I downloaded the same day and computer's all good. However on the next day, things that have never happened before are happening to my computer. On some websites, some of the words became hyperlinks(http://puu.sh/aB7dg/262489b2b0.png) and ads are appearing on places where there was no ads before. I thought that installing AdBlock might help, but it didn't. Also, a white gap keeps growing every few seconds between a Youtube Title and it's description(http://puu.sh/aB7p6/ec62f847b1.png) which goes on forever. I also noticed that my browser loads a bit slower now and sometimes doesn't even load the page. I'm getting the beach ball of death way more frequent now than before. Anyone know what's happening and how to fix this?

Answer 1

You probably installed the "DownLite" trojan, perhaps under a different name. Remove it as follows.
Malware is constantly changing to get around the defenses against it. The instructions in this comment are valid as of now, as far as I know. They won't necessarily be valid in the future. Anyone finding this comment a few days or more after it was posted should look for more recent discussions or start a new one.
Back up all data.
Triple-click anywhere in the line below on this page to select it:
/Library/LaunchAgents/com.vsearch.agent.plist
Right-click or control-click the line and select
          Services ▹ Reveal in Finder (or just Reveal)
from the contextual menu.* A folder should open with an item named "VSearch" selected. Drag the selected item to the Trash. You may be prompted for your administrator login password.
Repeat with each of these lines:
/Library/LaunchDaemons/com.vsearch.daemon.plist
/Library/LaunchDaemons/com.vsearch.helper.plist
/Library/LaunchDaemons/Jack.plist
Restart the computer and empty the Trash. Then delete the following items in the same way:
/Library/Application Support/VSearch
/Library/PrivilegedHelperTools/Jack
/System/Library/Frameworks/VSearch.framework
~/Library/Internet Plug-Ins/ConduitNPAPIPlugin.plugin
Some of these items may be absent, in which case you'll get a message that the file can't be found. Skip that item and go on to the next one.
From the Safari menu bar, select
          Safari ▹ Preferences... ▹ Extensions
Uninstall any extensions you don't know you need, including any that have the word "Spigot" or "Conduit" in the description. If in doubt, uninstall all extensions. Do the equivalent for the Firefox and Chrome browsers, if you use either of those.
This trojan is distributed on illegal websites that traffic in pirated movies. If you, or anyone else who uses the computer, visit such sites and follow prompts to install software, you can expect much worse to happen in the future.
You may be wondering why you didn't get a warning from Gatekeeper about installing software from an unknown developer, as you should have. The reason is that the DownLite developer has a codesigning certificate issued by Apple, which causes Gatekeeper to give the installer a pass. Apple could revoke the certificate, but as of this writing, has not done so, even though it's aware of the problem. This failure of oversight is inexcusable and has compromised both Gatekeeper and the Developer ID program. You can't rely on Gatekeeper alone to protect you from harmful software.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination  command-C. In the Finder, select
          Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.