Hello,
It seems that the two issues are unrelated, though life is full of surprises. I'd say let's treat them as separate issues for now.
Starting with the "login" issue: Just to make sure, are you selecting the "Remember me" option?
Also, make sure cookies are enabled; in IE8, go to Tools>Internet Options>Privacy tab. Check around in there, including the "Advanced" window.
The video issue seems like it may have to do with Flash. Make sure you have the latest version installed from here:
Adobe - Install a different version of Adobe Flash Player
What is the error you get when trying to play videos sent as an attachment?
One thing you may try is downloading Google Chrome from here:
Google Chrome - Get a fast new browser. For PC, Mac, and Linux
Chrome uses an embedded Flash player, while Internet Explorer and Firefox use a plugin.
Let's see where that gets us; if it doesn't work, we'll go from there.