The registry is a scary place unless you really know what you are doing. And I have heard many people say not to use a registry cleaner.
You are right that a celeron is maybe slower....but the computer is getting old too.
Try looking in the Control Panel and see if there is anything in there that you don't use anymore. Like an old game or something that you can remove.
Also on the right hand side it says remove Windows components..... I have done that a long time ago so I don't remember what I had unchecked.....I know one was MSN messenger......I don't use it so I dumped it. I use OE and IE so those were checked.....
And then there is adding more memory. My Dell laptop only had 512 MB of memory and I upped it to 2 GB and it made a lot of difference...
My 2001 desktop had only 256MB I went to Crucial to see if I could add more to it and they said probably not.....so I got a new desktop.....lol
http://www.crucial.com/ it is very easy to scan your computer to see what it says.