The wildcard only works together with the "Like" operator. You need to replace "=" with "Like".
Select Top 1000000 tblAssetCustom.AssetID,
tblADusers.Description,
tblADusers.Firstname,
tblADusers.Lastname,
tblAssetCustom.Manufacturer,
tblAssetCustom.Model,
tblAssets.AssetName,
tblAssets.IPAddress,
tblAssets.Domain,
tsysAssetTypes.AssetTypename,
tblAssetCustom.PurchaseDate As [Purchase Date],
tblAssetCustom.Warrantydate As [Warranty Expiration],
tsysAssetTypes.AssetTypeIcon10 As icon
From tblAssetCustom
Inner Join tblAssets On tblAssetCustom.AssetID = tblAssets.AssetID
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Inner Join tblADusers On tblAssets.Username = tblADusers.Username
Where tblAssetCustom.Manufacturer = 'Dell Inc.' And tblAssets.AssetName Like '%-%'
And tsysAssetTypes.AssetTypename = 'Windows' And tblAssetCustom.Warrantydate <
GetDate() And tblAssetCustom.State = 1
Order By [Warranty Expiration] Desc