2 reasons for this....
1. if you are using windows XP, and it is getting updated everytime u visit the net, then the internet settings get refreshed, and you have log in everytime you visit the site
2. your cookies settings need to be changed. make sure that cookies are enabled.
3. scan your PC for that freaking blaster worm which screws up a lot of things!!
try and copy and text from one of the posts in this forum, and then try pasting it in this quick reply box.....if you cant paste....then I am pretty certain you've got that blaster worm or maybe one of its variants.
In that case I suggest you go windowsupdate.microsoft.com-->critical updates-->blaster worm removal patch