Why does my macbook work better on a guest login account??

I have a 2007 macbook (pre-unibody) and it runs really really slow. A little while back I took it to the genius bar and they essentially "reformated" it with all the updates, etc. However, I just recently created a new login account and while I'm signed in with the new login account, the computer runs much more smoothly. Why do you think it's doing that?

Thanks in advance for all your help.

It's hard to say without a few more details but chances are you have software and settings impacting what runs at start up.

You can look at the items that load by going into System Preferences - Accounts - login items. Remove any you don't need/want.

Get hold of Onyx (appropriate to which version of OSX you have) and install and run all the items from the "Automated" tab.

This will clear outs caches, repair permissions, rebuild indexes etc. Basically an easy wash and brush up for your machine.