I suspect my iMac has been infected by a virus/trojan/worm/spyware. How can I search the hard drive

It's not a virus. It's a Web scam that only affects your browser, and only temporarily. There are several ways to recover.

1. Some of those scam pages can be dismissed very easily. Press the key combination command-W to close the tab or window. A huge box will pop up. Press the return key and both the box and the page will close. If that doesn't happen, continue.

2. Press and hold command-W. You may hear repeating alert sounds. While holding the keys, click the OK button in the popup. A different popup may appear, which you can cancel out of as usual.

3. From the Safari menu bar, select

Safari ▹ Preferences... ▹ Security

and uncheck the box marked Enable JavaScript. Leave the preferences dialog open.

Close the malicious window or tab.

Re-enable JavaScript and close the preferences dialog.

4. If the Preferences menu item is grayed out, quit Safari. Force quit if necessary. Relaunch it by holding down the shift key and clicking its icon in the Dock. None of the windows and tabs will reopen.

After closing the malicious page, from the menu bar, select

Safari ▹ Preferences... ▹ Privacy ▹ Remove All Website Data

to get rid of any cookies or other data left by the server. Open your Downloads folder and delete anything you don't recognize.

There is no currently known malware that can download on its own. "Cleaning" programs will only make your slowness problem worse. Please download and run EtreCheck, and post its output here. You can download EtreCheck from http://etresoft.com/etrecheck. Please don't post other kinds of diagnostics reports. They're very long and rarely helpful.

Check this link, http://www.tomsguide.com/us/best-antivirus,review-2588-6.html

There is no reason to download anything at all to solve this problem.

If Linc's steps are unsuccesful, force quit Safari. Do that by pressing the Command+Option+Esc keys. Highlight Safari and click the Force Quit button. Hold the Shift key down and relaunch Safari.

One other thing: All so-called "anti-virus" software is at best useless, and in most cases worse than useless. Even if it were not useless, it would not contribute anything to solving this problem, because you don't have a virus.

You have received some questionable advice in this thread. You posted:

My system is slow to turn on, tasks take longer to do & everything feels sluggish.

and the first reply you received is not related to that in the slightest, since you mentioned you already recovered from the pop-up. You are then told that you shouldn't download anything, which is generally a false statement, but particularly so when it comes from someone who does not understand what the problem is. You shouldn't download any anti-virus software, as it is not helpful and your problem is not related to that, but the claim made above referred to "[any] software."

Post an EtreCheck report as I previously recommended.

When you see a beachball cursor or the slowness is especially bad, note the exact time: hour, minute, second.

These instructions must be carried out as an administrator. If you have only one user account, you are the administrator.

Launch the Console application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad and start typing the name.

The title of the Console window should be All Messages. If it isn't, select

SYSTEM LOG QUERIES ▹ All Messages

from the log list on the left. If you don't see that list, select

View ▹ Show Log List

from the menu bar at the top of the screen.

Each message in the log begins with the date and time when it was entered. Scroll back to the time you noted above.

Select the messages entered from then until the end of the episode, or until they start to repeat, whichever comes first.

Copy the messages to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.

The log contains a vast amount of information, almost all of it useless for solving any particular problem. When posting a log extract, be selective. A few dozen lines are almost always more than enough.

Please don't indiscriminately dump thousands of lines from the log into this discussion.

Please don't post screenshots of log messages—post the text.

Some private information, such as your name, may appear in the log. Anonymize before posting.

When you post the log extract, you might see an error message on the web page: "You have included content in your post that is not permitted," or "The message contains invalid characters." That's a bug in the forum software. Please post the text on Pastebin, then post a link here to the page you created.

You are welcome.