I installed Windows 7 yesterday on my hp pavillion a1217n (Celeron) just to simply take a look at it. It worked great but for some reason it seems that my computer is not able to cope up with windows 7. I reinstalled windows xp on this computer yesterday. My computer is almost done. I can't wait to install it on my new computer.
What exactly is your celeron processor? Is it a netburst celeron D or celeron dual core or something else...? And what is its frequency?
Because newer celerons (E1000, E3000 series) should be more than enough to handle windows 7, older ones can be a little slower.
And how much ram do you have? If it is 1 GB or less it can be the cause of slowdown under windows 7.