The message you saw may have come from scareware on your system, i.e., software that attempts to scare a user into buying a product by claiming that his system is infected by a lot of malware or makes other false claims. Or, if you are seeing the message in your browser, it may simply be that you've visited a website that is displaying a similar message to convince you to buy a product or call a phone number where you will be asked for a credit card number to pay for a "technician", who may claim to work for Apple, Microsoft, or some other company the user would likely think is a legitimate company, to fix your system, whereas, in reality, you would be providing a credit card number to a scammer who would "fix" a nonexistent problem and then might not only charge the cost for his "service" to your credit card, but use your number for other charges or provide your credit card information to others who would do so. You didn't provide a screen shot or enough detail for others to determine whether the former or the latter possibility is more likely, but I've encountered a number of cases recently where users have been scared by such messages being displayed from a webpage where the only action needed was to close the tab in the browser for that particular webpage. If you closed your browser and never saw the message repeated, then I'd suspect the latter possibility explains what you saw.
However, for peace of mind, some suggestions for legitimate, free antivirus/antispyware products you could use to scan your system, which shouldn't conflict with an existing antivirus/antispyware product on your system, that can be run manually on-demand, i.e., rather than having them constantly monitoring your system causing continuous additional CPU and memory load:
- Ad-Aware Free Antivirus+
- Malwarebytes Free
- Sophos Virus Removal Tool
- Spybot - Search & Destroy
I've sometimes found that using several antivirus/antispyware products will allow me to identify malware that is relatively new or particularly insidious. It isn't always that one product is far superior to all others, but, instead, that the developers of some products have not yet encountered a new malware variant, but one particular company or organization has already encountered it and has updated their virus/spyware definitions to detect it. So for malware x, product A, B, C, and D may miss it, but product E may find it while for malware y, A, C, D, and E may not identify it, but B does. When I've had the time, I've sometimes tried a dozen or more before finding one that detected a particularly insidious piece of malware. But, unless the message is popping up, even after a scan by just one, such as Malwarebytes, declares your system to be free of malware, a scan by just one of them is probably enough for assurance that your system isn't actually infected/compromised.