One of the main arguments against software patents is that it reduces innovation and generates unnecessary litigation. Microsoft has been suffering considerably from software patent litigation, perhaps because they are the largest and juiciest target for patent trolls and for patent owners interested in reducing MS market power.
Symantec is suing Microsoft in order to obtain an injunction to stop development of Windows Vista, as they claim that Gates & Co are using Veritas storage technology, for which Symantec owns the patents obtained from tech company called, unsurprisingly, Veritas.
Hopefully, the software development scene will become so difficult to navigate that large companies will lobby for reform of the American patent system. If you want to have a look at the picture is in Europe, you can read this article here *cough*shamelss plug*cough*.