At this stage of development at any rate, this is not the case. The success of the PC (as opposed to the Mac and IBM's second unsuccessful go at cornering the market, the PS/2) was that it was based on standards that lots of people could both copy and make standard parts for.
In many other areas of computing - networks are a good example - all vendors make a big sales pitch of adhering to standards because they know buyers typically have a mixture of gear from different vendors which must interoperate, and also want to retain the option of changing vendor. Most vendors have added features which are proprietary, but buyers use those at their peril.
Such standards are appearing thick and fast ("the advantage of standards is that there are so many to choose from"!), developed and agreed on by committees generally heavily staffed by vendor representatives.
Of course when a vendor feels it has sufficient dominance it starts ignoring or adding proprietary features to standards to lock customers in - that is what Micrsoft is doing to the Java standard. So all this cooperation may be just a transitional phase, but it is alive and well in many areas untouched by Microsoft, for now anyway.
Bill
Bill Rosenberg, Deputy Director, Computer Services Centre University of Canterbury, Private Bag 4800, Christchurch, New Zealand w.rosenberg at csc.canterbury.ac.nz. Ph 64 3 3642801. Fax 64 3 3642332. Room 211, Ext 6801