It is interesting that you say that. I had exactly the same experience 3 weeks ago when I got my new Q6600 Intel Quad with 4GB's of RAM. I expected it to be lightning fast, but there was also a lot of donut spinning. In fact it was a lot slower than my AMD 4800+ with 3GB of RAM.
But now, after a couple of weeks, it runs a lot better. I think there is a learning process going on (especially for Superfetch) and things get in order after a while. Changing back to XP will probably not help. In fact I would suspect it to be worse.
There are, of course, tons of tuning tools available. All you need to do is Google "Vista tune up" or something like that, and you will find tons of them. I would, however, be careful. A lot of those will do more harm than help. In my case, I did nothing. Just stuck in a 4GB USB stick for Ready Boost hoping to help with the paging. My page file is about 7GB. But I am not even sure whether that really helps.
Bottom line: Wait a couple of weeks. Things can only get better.