If you're using the latest version of IE (IE8) then multiple processes is to be expected. IE8 starts a parent process and then loads each window and tab in a separate child process. This improves the responsiveness and robustness of the browser.
If, however, you are not using the latest version, then please post back. (Also, more detail would be helpful. Like, does it only slow down after visiting a certain site?)
Edited by Amazing Andrew, 06 October 2009 - 12:37 PM.