I am running Windows 7 and using Internet Explorer 8 and for the last few days have been unable to login to Garmin Connect. I entered my username and password but received a "security" error message. I could see that I was logged in but couldn't access my activities or dashboard. I tried all the usual workarounds of adding Garmin Connect to my trusted sites etc. but nothing worked. I installed Moxilla Firefox and was able to access Garmin Connect fine with that, so gathered it was an IE issue.
I then tried this :-
https://forums.garmin.com/showthread.php?t=287
Even though this is specifically for Windows Vista it works for Windows 7 too.