It's either a false alarm or a Windows malware attachment in email (a common occurrence for most email users.) Neither one can harm you, and you don't need to do anything about it. Just don't pass it on to anyone else.
What you do need to do is get rid of the "anti-virus" (AV) software. All such software is worse than useless.
Windows malware is so widespread that you should assume it's in every email attachment until proven otherwise. If you're just curious as to whether a file is recognized as malware by AV engines, you can upload it to the "VirusTotal" website, where it will be tested against most of them. I don't recommend doing this with a file that might contain private information. A negative result is no proof of anything, because AV software is not reliable.
Never allow any AV software to delete or "quarantine" email messages or attachments. Doing so will corrupt the Mail database.