You've added tsysOS, but you didn't link it, so you are multiplying your number of records in the report by the number of records in tsysOS this way.
Please try this first:
Select Top 1000000 tblAssets.AssetID,
tblAssets.Domain,
tblAssets.Username,
tblAssets.IPAddress,
tblAssets.Description,
tblRegistry.Value,
tsysOS.OSname,
tblComputersystem.SystemType,
tblAssets.Userdomain,
tblAssets.Lastseen
From tblAssets
Inner Join tblRegistry On tblAssets.AssetID = tblRegistry.AssetID
Inner Join tblComputersystem On tblAssets.AssetID = tblComputersystem.AssetID
Inner Join tsysOS On tblAssets.OScode = tsysOS.OScodeWhere tblRegistry.Value Like '%6.0' And tsysOS.OSname Like '%Win 7' And
tblComputersystem.SystemType Like 'x86%'