Microsoft does have issues with scalability in their desktop products, and while most of the operating environment is modular, the relationships between various components aren't well documented.
I think we also need to consider the state of the hardware market. CPUs today are pretty much the same speed they were five years ago, and not every software problem benefits from parallel processing, larger caches, higher bus speeds, and the other changes the industry has made to mask the performance cap.
Salespeople, particularly at department and electronics stores, are usually just poorly educated. They base their recommendations on marketing and sales incentives. Spending sixteen hours a day in World of Warcraft does not turn a salesperson into a PC expert. ;-)
EDIT: And the fact that everyone assumes "PC" means "Microsoft Windows running on an x86[-64]-compatible processor" is evidence of the success of Apple's attempts to differentiate themselves. "PC" used to mean all personal computers, IBM, Apple, Commodore, Atari, et al included.