I think a trojan virus has infected my iMac. How can I remove it?

Question

My iMac has been acting weirdly opening new tabs on my internet explorer (usually betting or **** stuff). I have downloaded ClamXav and ran a few scans but nothing other than finding emails with the virus (which I have obviously deleted).
Now one on my mailbox has been acting weirdly too in that I can't connect. I tried to connect via a webmail and the password wasn't working. I then changed the password on the control panel for the same mailbox and it works on all my devices (phone, iPad) but each time I try to change the password on my iMac, nothing... and then within a few minutes my password doesn't work on my other devices either.
I have now deleted the mailbox in despair but I guess that doesn't clean my Mac.
I also downloaded the OS Yosemite today int he hope the upgrade would clean my Mac.
Any suggestion? software I can buy to scan my iMac. I know Symantec is not compatible.
Thank you.
C.

Answer 1

You may have installed the "VSearch" trojan. Remove it as follows.
Malware is always changing to get around the defenses against it. These instructions 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 before proceeding.
Step 1
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," "Trovi," 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.
Reset the home page and default search engine in all the browsers, if it was changed.
Step 2
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 "com.vsearch.agent.plist" 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
Restart the computer and empty the Trash. Then delete the following items in the same way:
/Library/Application Support/VSearch
/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.
The problem may have started when you downloaded and ran an application called "MPlayerX." That's the name of a legitimate free movie player, but the name is also used fraudulently to distribute VSearch. If there is an item with that name in the Applications folder, delete it, and if you wish, replace it with the genuine article from mplayerx.org.
This trojan is often found on illegal websites that traffic in pirated content such as movies. If you, or anyone else who uses the computer, visit such sites and follow prompts to install software, you can expect more of the same, and worse, to follow.
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 Internet criminal behind VSearch 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 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.