I think between the overly-agressive memory management of windows (6GB of RAM, 1.5GB in use for active programs, 2GB for system cache, and it's *still* paging stuff? WTF?), and the fact that it memory-maps executables (as opposed to copying them to memory to run) cause major problems if a drive is operating slow. True it can service another process while it's accessing the disk, but what if that process has to be paged off of disk before it can be run? True the CPU can go onto the next process, but I'm still here waiting for Word or WoW to open. Memory-mapping of executables was great when all we needed was 640Kb of memory, but today it's only purpose is to require exclusive filelocks when a process is running and prevent the application from being updated. This doesn't explain why a system would run fast for weeks/months and then suddenly slow down one day, but it's a serious flaw in how windows manages it's memory and processes.

See, the beauty of multi-process (or multi-threaded, even) is when one tab locks up, the rest of the browser doesn't. Open Chrome's process manager (Shift+Esc) and kill the tab. And I don't know anyone that goes around trying to open tabs that will lock up the browser. That's done on accident, and it's nice to be able to do something about it when that occurs.