Back when we our company had an Altiris AE, I asked him about this. He went to the developers and came back and said the ASDK method was specifically made to apply only to guids who are of the type "collection". I think our AE added a feature request to allow computer guids, but I can't find any KB article to reference this. I will look through my old e-mails and see if I can find it.
To work around this, I create a collection, add the computers I want to the collection, update the collections, execute the task, then delete the collection. The tasks work fine if the collection is deleted AFTER you run the ExecuteTask method. The whole process is so fast that you never see the collection in the system. It's a dumb that we need a workaround but hopefully they will address if/when ASDK 1.5 releases.