Early on I had a tendency to just install and uninstall willy-nilly. It wasn't until I ran into problems that I learned to manually clean up after an uninstall, make sure all the unnecessary files are deleted, to check for the latest drivers (even for software), and yes, even occasionally to clean the registry. Also make sure new programs don't automatically load on startup. (Unless you really want them to run at startup -- but do you really need photoshop to run at startup so that _if_ you want to edit a photo it will pop up in an instant instead of taking a minute to load? So you're willing to wait an extra minute for startup every time you boot up? What about all the other programs on your computer? Do you need them all running in the background, eating up all that memory [however much it is]?)

That's the real reason Windows is unstable (oftentimes): people who don't know what they're doing install stuff that "helps" them by doing things automatically, and it all piles on. (There's a mathematical theorem that if you put strings in a drawer, they will knot themselves [well, that's not the mathematical wording, but the translation into everyday words -- and proof that knot theory is useful: it's not just bad luck that those spare shoelaces tied themselves together but _logical inevitability_]. Similarly, if you just put complicated programs together, they will mess the computer up (note: not proven, just asserted -- computer programs are not exactly the same as shoelaces). They have to be managed.