Very few people are going to go through the hassle of installing a new client just to make a phone call if a good solution, which both parties have, already exists.

Well, they did that for Skype. And if it comes with the platform, that barrier is gone.

But yes, there is market inertia and Skype has it.

Most people I know who use Skype didn't have a video/computer calling solution before Skype. The barrier to installation for a new service that is much lower than switching to a competing service. If they're just getting into that kind of service, the benefits are quite large. Switching to a competing service has much less utility to them, because they already have a solution for the main problem; in this example, calling people. Switching requires losing their old contacts (network effect) as well as learning a new system. To put it in equation:

Installing something like skype for the first time:

Total utility of program - hassle of installing and learning a new program = large benefit

Installing a competitor:

Marginal utility of somewhat better program - previously available contacts on old system - hassle of installing and learning a new program = much less benefit

I think you are overestimating the average computer user. As personal computing and broadband access has risen quickly, so have the class of technophobes.

Also, a product will seldom sell itself. The Skype brand has plenty of intangible value, for instance from their collaboration with Oprah.