I was encountering the same problem. The second user account that was created (admin account with Windows XP), IE8 would be very slow starting up.
What I done was change some of the Search settings.
In IE8, click on Tools, then Internet Options, then on the General Tab, for Search it was then settings. It was mainly disabling some of the add-ons. Under Toolbars and Extensions, I disabled a few of the items that I never used, I kept most Google items, Java stuff and Norton Stuff. Under Accelerators, I removed a bunch from there that I haven't used... and not using InPrivate Filtering. Then I kept all Lan Settings unchecked.
After all was done, restarted the computer logged on the the main account, then switched to the second user account, and things started up much faster than before.
It worked for me at least....