(1) Are you using the secure https:// login?
http://en.support.wordpress.com/https/

(2) Exactly which browser and version of it are you using. If you aren’t sure click here to find out > http://whatsmyuseragent.com/

Note: If you are using an IE9 browser in compatibility mode that makes it render as IE7 and IE7 is not supported. To disable see > http://www.sevenforums.com/tutorials/1196-internet-explorer-compatibility-view-turn-off.html

Have you done any troubleshooting?
a. clearing your browser cache and cookies > http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=32050
b. Using the browser with all extensions or add-ons disabled
c. updating the browser to the latest stable version http://browsehappy.com/
d. Trying the other troubleshooting images tips http://en.support.wordpress.com/images/troubleshooting-images/