If you have a non-genuine copy of XP then you won't be able to install IE7. It's no real problem - get Firefox, it's better than IE anyway.
If you're getting a lot of pop-ups then chances are you are infected with malware. This could also be the cause of your Buffer Overrun errors.
Try scanning with your AVG antivirus in safe mode. Also, install some more anti-spyware applications and scan with those in safe mode also. I would recommend:
These are all free and links can be found in the following thread:Freeware Replacements For Common Commercial Apps
Also try a couple of the online scans listed in the above thread.
That should keep you busy for a while. If you still have problems get back to us.