well, i wouldn't go as far as to say that Windows is total garbage. you can play some nice games on it, and since XP it's been solid as a rock for me unless hardware failures are counted (which they aren't). i have an issue with the way that it allocates resources, and with the file browser being integrated into the GUI in such a way as to cause the GUI to have to restart if the file browser hangs, but other than that the only really major problem i have is security.
Despite the fact that I'm a Linux geek, I have to agree. However, I have managed to have 2k and XP crap out on me and I couldn't fix it. The Windows allocates its resources is infamous anyway, and I've decided that GUIs are nothing you can discuss anyway because everyone has personal preferences there. So, Windows isn't bad, save for two things which continue to annoy me:
Security. You mentioned it and it could be fixed quite easily. Linux isn't some sort of magical-super-safe-fairy-land-OS, the reason it's safer is because you're usually not logged in as root. Why can't Windows implement a decent user management and rights system and use it for once? That would make Windows a hell of a lot safer just like that.
Also, the fact that Windows has no decent error log. If your system starts acting up, it's incredibly hard to find the source of it because you can't check any good logs.
seeing as Vista is just a refinement of XP i think that it'll be a decent OS. (unless they've realy dicked around with it and made it unstable)
As Nexus mentioned, the codebase was Win2k3 Server, so I doubt it will be unstable.
However, Vista is, architecture-wise, a new OS. Have a look at their Nexus-kernel-architecture, that's a radically different from anything else they've done before.
Also, as I mentioned, integral parts of the GUI have changed and a new user/rights-management system has been implemented. Not mentioning TCPA support here, which is also new.
In short, this is not an XP-upgrade (that wouldn't take Microsoft 5 years either). It's a new OS.
And I don't call a system rock solid unless I've managed to have it run (and update the system itself without rebooting) straight for more than 100 days. Which works with Linux (Slackware, in case you want to know).
Oh, and Nexus' last point is good. There would be two reasons I could see: Microsoft ends support for XP and an improved user/rights-management.