See if this works for you:
Select Top 100000 tblAssets.AssetID,
tblAssets.AssetName,
tblAssets.Domain,
tsysOS.OSname,
tblAssetCustom.Manufacturer,
tblAssetCustom.Model,
tblAssets.Memory,
tblAssetCustom.Serialnumber
From tblAssets
Inner Join tblAssetCustom On tblAssetCustom.AssetID = tblAssets.AssetID
Inner Join tsysOS On tsysOS.OScode = tblAssets.OScode
Where tsysOS.OSname = 'Win XP' And tblAssetCustom.State = 1
Order By tblAssets.AssetName