Please Help! Back office login problem

[smiledog]

I've been using Prestashop happily for 6 months now, everything was fine until recently I installed Kaspersky internet security, at first I can't log into back office using Firefox, but Chrome is okay.
I just changed ISP and uninstalled Kaspersky internet security, now even Chrome won't let me log into back office, after I entered the username/password, it jumped right back to the login page, I have to enter my username/password a few times to get into back office, once in the back office, I clicked an order, it jumped right back to the login page, I have to enter the username/password again and agian.

I tried Firefox, Chrome, and Safari, all the same situation, can anyone help???