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Factory condition? You're lucky if Vista even works in "factory condition!" Why not just remove your personal files (which most people put under MyDocuments and MyPhotos and such) and leave it at that?
I GUARANTEE you, if you start removing this and that piecemeal, you're going to end up with more problems than you know what to do with.
"Reformatting" will destroy the hard drive and you'd have to reinstall the operating system, utilities, etc. Any customizations you had performed would also be gone.
Had you a backup of the settings that were there when you took the machine out of the box, you could reset to that state--although it's not trivial to achieve.
Otherwise, your best bet is to remove your personal files (use an erasing utility like Eraser: please stay away from BleachBit). That will also preserve handy customizations to the environment that you've effected over the eons. "What customizations?" you say. You've probably tinkered with dozens of things that you just don't recall offhand!