As the Report Builder is an SQL Editor, you can e.g. make use of the '%' and '_' wildcards in combination with the 'LIKE' statement. More information on the use of the wildcards can be found
here.
The following example filters the assets on the asset type containing 'win':
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetName,
tsysAssetTypes.AssetTypename,
tsysAssetTypes.AssetTypeIcon10 As icon,
tblAssets.IPAddress,
tblAssets.Lastseen,
tblAssets.Lasttried,
tblAssets.OScode
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Where
tsysAssetTypes.AssetTypename like '%Win%' And tblAssetCustom.State = 1