I thought there was one already (I have one) that is a canned report but maybe I added it a while back and forgot.
I assume you mean something like this....
Select Top 1000000 tsysOS.OSname,
Count(tblAssets.AssetID) As Total
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysOS On tblAssets.OScode = tsysOS.OScode
Where tblAssetCustom.State = 1 And tblAssets.Assettype = -1
Group By tsysOS.OSname
Order By Total Desc