I am having the same problem.. I registered.. no error messages. When I try to login it just takes me back to the main page with no error messages and I am not login in.
Any chance you are hosting with dreamhost? I have more problems with them than with any other host I have been with.
I did not install zencart.. dreamhost now has a one click installment for it. So I am wondering if some permissions were not set correctly with their one click installment.
Also... how do I get rid of this below on my main page:
Congratulations! You have successfully installed your Zen Cart™ E-Commerce Solution.
http://www.psptuts.com/cart/
Deb