If we understood you correctly, you want to have a report that will give back machines that are newly scanned (and are not already in the Lansweeper database) where no AntiVirus information is found. Afterwards, you would like to have an email alert send for it whenever there is a PC found without antivirus information. More information on how Lansweeper scans anti-virus information can be found in 
this knowledge base article.
There are two built-in reports that are called Workstation: All workstations without Anti-Virus software and Server: all servers without Anti-Virus software that can be used to base yourself on. For your convenience, we have added a custom sample report below, that will give back both workstations and servers that are recently added to the Lansweeper database (first seen date 
smaller than 3 days). You can change the report in this way that it will only give back assets that are first seen in the last day or the last week by changing the number 
highlighted in green below.
After you have added the report to your Lansweeper installation, you can set up an email alert under Configuration\Email Alerts by following the steps in 
this article. Keep in mind that the email alert will only be sent if the report has results. Instructions for adding this report to your Lansweeper installation can be found 
here. If you are interested in building or modifying reports, we do recommend:•Reviewing some SQL tutorials, as the Lansweeper report builder is a standard SQL editor. If you know SQL, you know how to build Lansweeper reports as well. 
This seems like a good tutorial.
•Making use of our database dictionary, which explains in great detail what each database table and field stores. More information on the dictionary can be found 
here.
Select Top 1000000 tsysOS.Image As icon,
  tblAssets.AssetID,
  tblAssets.AssetName,
  tblAssets.Domain,
  tblAssets.Username,
  tblAssets.Userdomain,
  tblAssets.IPAddress,
  tblAssets.Description,
  tblAssetCustom.Manufacturer,
  tblAssetCustom.Model,
  tblAssetCustom.Location,
  tsysIPLocations.IPLocation,
  tsysOS.OSname As OS,
  tblAssets.SP As SP,
  tblAssets.Firstseen,
  tblAssets.Lastseen
From tblAssets
  Inner Join tblComputersystem On tblAssets.AssetID = tblComputersystem.AssetID
  Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
  Inner Join tsysOS On tblAssets.OScode = tsysOS.OScode
  Left Join tsysIPLocations On tsysIPLocations.StartIP <= tblAssets.IPNumeric
    And tsysIPLocations.EndIP >= tblAssets.IPNumeric
  Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Where tblAssets.AssetID Not In (Select tblSoftware.AssetID
  From tblSoftware Inner Join tblSoftwareUni On tblSoftwareUni.SoftID =
      tblSoftware.softID Inner Join tsysantivirus On tblSoftwareUni.softwareName
      Like tsysantivirus.Software) And tblAssets.AssetID Not In (Select
    tblAntivirus.AssetID
  From tblAntivirus) And 
tblAssets.Firstseen > GetDate() - 3 And
  tblAssetCustom.State = 1
Order By tblAssets.AssetName