for windows based systems:
click on your control panel then click on "internet options", under the tab "General" there will be a section called temporary internet files, i believe there will be a delete cookies button if i remember correctly (i'm running a unix based OS so i'm not 100% sure, but i am 99%) delete the cookies and then after that click on the button to the right of delete cookies called delete files, this will clear our your temp. internet files. then try and logon to the website.
if your computer illiterate i'm pretty sure your not running a unix based OS so i wont go into that, it's prob windows. if its an OS/2 type OS let me know and i'll walk you through it. ahh, my tech support days, hehe, too bad i aint gettin paid for it no more