Could be a lot of things, but my guess would be the bulk of it has to do with the necessary protections for Windows. You quite probably have one of the more hefty AV programs like Norton or McAfee which toss in everything short of the kitchen sink. You might have also picked up some malware along the way that isn't helping, and any other programs you might have installed that launch at boot won't help your cause either.

Those are just a couple of areas to get you started.