"Hi name-withheld.

We installed a free product called ClamXav on your big Mac when you had the Adware problem a while ago.  You can run a scan using ClamXav to check for and clean up any problems on that Mac.  

Given that you alsohave  a Macbook, you will need to install the same (or similar) product on that device.   ClamXav is a free app in the Mac App store, so you can download it from there.  Here is a link if you want further details.

But I am not convinced that one of your computers is the cause of this particular problem.

Typically, your Paypal account would not be the only thing ‘hacked’ if you have some virus, malware or spyware problem on one of your computers.  Have you had problems with any other financial, iCloud, or email accounts?

The problem will usually be that you have inadvertently given your email address and password away by clicking on a suspect link in an email - which could come through on EITHER your computer or you iPhone/iPad (which does not have any anti-virus product, and does not need one).  These emails often come from someone you know, so may not be blocked by any anti-virus software on a computer.  

You need to be vigilant about never clicking links in emails that you receive - unless you are ABSOLUTELY sure of the validity of the email.  

PayPal, banks etc will never contact you directly and ask you to click on a link.  They will ask you to go to their website and log in as you usually do.  As a general rule NEVER click on an email link.

Have you changed your PayPal password to something more secure, and one that does not match any other account password that you use?  

You really should think about turning on the on two-factor verification feature offered by Paypal, called Security Key. 

You can set this up to send a security code to your mobile phone whenever someone accesses your account.  So this stops anyone that is not you from getting into your account.

Given that you travel a lot, this can also be a problem as, if you use public and hotel WiFi on a regular basis, you can have your details stolen by ‘WiFi sniffers’ who look for any un-encrypted communications across that public WiFi network.   If you tend to frequently use public WiFi, you should use a VPN (Virtual Private Network) to ensure all your online communications are protected.  Several are available - I use one called Private Internet Access when I travel.

In terms of your question about the best antivirus software for Mac, check out this article from Macworld and this other one from PC Magazine.  If you decide you need a different product to ClamXav, you would need to install this software on both your iMac and your Macbook.

Once again though, I can't emphasise strongly enough how important it is to have different strong passwords for each of your important online accounts (banks, iCloud, eBay, PayPal, Gmail, Hotmail, Outlook, other email accounts, etc) and, wherever possible, utilise the two-step verification feature that those online accounts all offer.

Hope this helps."