Q: How do I now if I have a malware or virus?

The pop-up is just a simple Javascript and is trying to scam you.

And no, there's no virus in the wild that will infect your Mac.

If you stay vigilant and keep your Mac OS X up to date, you'll be fine.

Apple nor Safari warns you for malware, viruses or whatever. Even more, no website is able to scan your Mac for malware or viruses.

If the slowness doesn't repeat itself, I wouldn't worry. Personally I don't think the two events are related.

dvniellin wrote:

 

Yeah, but if nor apple or safari does this, that means that the site was trying to caught me in some kind of malware or virus

 

but as seen as I don't have Java installed or didn't found anything to alarm me off while cleaning the cookies, caches, download and history I can be cool?

 

I' m just still concerned because right after this event my mac started to working slowly and I couldn't do anything until I restarted my mac

It wasn't trying to catch you in some kind of malware or virus.  For one, there are no viruses for OS X.  Two, the kind of malware that Mac users can get is downloaded and installed by you, usually through downloading something via torrent or aggregate download site, as those installers many times are bundled with malware and adware.  Three, it's a scam, as ckuan pointed out — it's a simple browser hijacker, and they're easy to dismiss.  Force Quit Safari ( --> Force Quit --> Safari), then restart Safari in Safe Mode by holding down the Shift key.  In the future, stay away from the kinds of sites where scams like that reside.

You're welcome.