How to know if I have virus/whatever

There are no viruses affecting OSX in the wild. And it's unlikely that you have been hacked - it's far more probably that someone has picked up your email address and is sending spam using it as the 'from' address; this is a very common practice. That's how you've been getting emails apparently from yourself - spam with Zip files is again common, though usually it claims to be a invoice or some other document. In fact it's malware, though probably only affecting Windows - don't open it, obviously.

It's infuriating when someone uses your address in this way but there's not really anything you can do about it. Usually it stops after a bit as the spammers move on to some other victim's address.

Roger's note is indeed very common. Spammers pick up addresses from all over the place, and even sell lists to each other. They could have gotten yours from any number of web sites you were required to enter your email address at.

Another way is Windows email viruses. I've also gotten some from "myself" that I know actually came from one of my brother's computer. There are hundreds of these things that exploit Windows. An infected email comes in. Some launch the virus as soon as it hits the inbox. Others have to wait until you click the attachment. Either way, it then randomly selects a name in that person's address book to spoof as the sender, and then sends out the same email to every person in that list.

This can't happen on a Mac as these emails viruses require the Windows OS to work. They're nothing on your end. If you get one, you can simply delete it. The "ex friend" is blaming you from something you didn't do.

No hacking is necessary to perform this, you simply need an email address, not anything else.

If the spamming is not specific it is likely something that could be perpetrated by someone who has no knowledge of your existence. but if it is specific in it's destinations it's very likely someone who has a specific agenda against you and/or the ex friend.

If you've explained to your ex-friend this is not something you would consider under any circumstance that is all you can do short of examining one of those emails for the header info for where the mail was sent from and determine what mail relay it was sent from or if it was sent via a proxy. While a proxy will hide the IP the site and other sender info the site URL itself could still be located in the perpetrators browser history.

I don't use windows...at least not that I know of??

Not yourself. Anyone you know who does use Windows and has your email address in their address book.

Any one of them could have gotten a Windows email virus, which then sent that same email out to everyone in their address book. As mentioned, these viruses randomly pick an email address in that address book as the "sender". That's one way to get an email seemingly from yourself.