I think TM is what you put prior to the mark being approved as a registered trademark by the USPTO. (It's analogous to "US Patent Pending" but for trademarks.) So eventually there will be two (R) on that logo.
Maybe Microsoft could make the whole logo out of (R) symbols. Then the ones inserted by the lawyers will fit in.
TM doesn't mean anything - a trademark is either registered (and therefore enforceable) or it isn't. You can slap it on pretty much anything you want, it doesn't mean that whatever it is attached to will become a registered trademark, or even that the owner has filed for registration.
I can't speak for the USA or anywhere else but in Canada TM means something. Common law trademarks do come with some rights to protect your brand, Registered trademarks come with more. It's been a long time since my IP Law class but I beleive TM allows protection against business in the same industry from using your trademark in such a way that it could be reasonably confused for yours.
TM still doesn't mean anything in a legal sense - if you're relying on the common law protection then it doesn't matter if you put the two letters next to your logo/brand. Putting TM on doesn't mean you have any protection, and leaving it off doesn't mean you have none.
It suggests that the company has some rights to the brand and might act as a deterrent to others using the brand without permission (especially as most people probably don't understand the difference between registered and unregistered marks).
Maybe Microsoft could make the whole logo out of (R) symbols. Then the ones inserted by the lawyers will fit in.