Actually, MS more or less invented the tablet PC. Gates seems to have had a old dream about pen based computer input, and so they came up with the UMPC and tablet pc concepts and presented them around 2001. Problem was that the CPUs at the time too power hungry, and so one could not really get any work done. There was also some resistance from inside MS, resulting in MS Office lacking integrated pen support. So rather than notating directly into Excel or Word, people had to make do with a input box that was really meant for "legacy" programs.
And i think OSX picked up steam in CS and various science fields because of the UNIX/BSD core. This especially once it ran on x86.