all the suggestions sounds great , you need something unique that restores or at least fixes some security settings that have been changed for a reasons which it could be due to a malware , id suggest trojan remover from www.simplysup.com , you can get your 30 days trial but probably your going to use it once , run a scan , not afull scan but a smart quick scan , and remove the infections and hidden drivers and kits that is infesting the system and its going to tell you that something changes some security setting son the system do you want to undo that so yes is the answer then a final reboot

i had that exact problem once and i did it with TR and then running sfc /scannow to make sure all the system files are available and not corrupted then reinstalling IE8 and ran a chkdsk /p from recovery console " using a windows xp cd"

and the last thing i would say is good luck