Malware

Assuming 10.10.2 was properly installed, boot while holding down ⌘R to boot into the Recovery Drive. From there you can erase the drive and install Yosemite from the internet.

Malwarebytes is free, does not install anything untoward, does a good job of identifying and removing lots of adware and other malware, and in general is safe to use. It might solve your problem cleanly.

Flashmall is adware you installed along with one of third party apps you mention. It's especially likely if you downloaded them from Softonic.com, or C|NET's www.downloads.com.

Adware is basically just REALLY annoying, and slows your computer down with the enormous amount of system resources they use up retrieving ads from remote servers and then displaying them anytime your web browser is open.

Erasing the drive is overkill for adware. Open Safari's preferences and click on the Extensions tab. Turn them all off. Turn them back on one at a time and test. When the ads come back, you'll know which one is the culprit. Turn it back off and delete the extension. Repeat the same test for all of them as you may have installed more than one adware extension.

You installed the "Flashmall" trojan. Take the steps below to disable it.

Malware is always changing to get around the defenses against it. This procedure works as of now, as far as I know. It may not work in the future. Anyone finding this comment a few days or more after it was posted should look for a more recent discussion, or start a new one.

Back up all data before continuing.

1. Triple-click the line below on this page to select it, then copy the text to the Clipboard by pressing the key combination command-C:

~/Library/LaunchAgents

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. A folder named "LaunchAgents" will open.

2. Inside the folder you just opened, there may be files with a name beginning in any of the following ways:

com.crossrider

com.extensions

com.flashmall

com.Installer.completer

com.webhelper

com.webtools

flashmall

UpdateDownloader

WebSocketServerApp

Move any such files to the Trash and close the Finder window. Log out or restart the computer. The trojan will now be inactive, but there are a few more components of it that should be cleaned up.

3. Do as in Step 1 with this line:

~/Library/Application Support

A folder named "Application Support" will open. Inside it there may be subfolders with any of these names:

IM.Installer

webHelperApp

WebTools

If so, move those subfolders—not the "Application Support" folder—to the Trash.

4. Open this folder in the same way as above:

~/Library/ScriptingAdditions

and remove an item named

BrowserHelper.osax

if present.

5. Open this folder:

~/Library

Look for subfolders with either of these names:

flashmall

WebTools

and move them to the Trash, if present.

6. Open the Applications folder. If it contains an item named "Flashmall" or "WebTools", move that to the Trash.

Important: You can't delete applications by trying to drag them from the Dock or the LaunchPad. Open the Applications folder in the Finder.

7. Open this folder in the same way as above:

~/Applications

This is not the usual Applications folder, but a different one inside your home folder. Look for an application with a name like this:

flashmall

and move it to the Trash, if present.

Empty the Trash.

8. From the Safari menu bar, select

Safari ▹ Preferences... ▹ Extensions

Uninstall all extensions you don't know you need, including one called "GoldenBoy," if it's present. If in doubt, remove all of them. None is required for normal operation. Do the equivalent in the Chrome and Firefox browsers, if you use either of those.

Only if you can post links to what you downloaded.

Do not remove WebKit it is installed as a part of OS X and is essential to the operation of Safari and other parts of the OS.

clicked on the links to "get express VPN now"

I don't see any way to download the software without signing up for the service, which I'm not going to do, but I doubt that the trojan came from there.