Windows XP and Windows 7 are sold bundled with computers - you basically can't buy one without the other. The case of Vista is different - it was so awful people bought computers with Vista (counting as a Vista sale) and upgraded them to XP. Low sales indicated people put off buying new computers because it wasn't worth it - the new computer with Vista would be worse than the old one with XP. Windows 7 success was caused, in part, by those computers - they wouldn't last forever and, one day, would have to be replaced. Windows 7 was being bundled by then. And it was good enough as not to warrant upgrades to XP.
Office has no credible competition. Weren't various versions of Office using undocumented Windows APIs that their competitors couldn't and, thus, had lower performance and worst integration with the environment? I remember something in these lines. That and bundling it with Windows in corporate licenses that made others non-competitive.
And, please, Sharepoint is a joke. The only reason someone still uses it is because it's impossible to migrate its contents to any sane platform. And, of course, fear of admitting such an incompetent decision choosing it.
As for SQL Server, it's a decent platform. I have to admit it is Microsoft third best product, behind only the natural keyboard and their mice.