No matter what you have heard there does still seem to be problems with IE8 and XP although it seems to work well with Vista.
Try uninstalling IE8 and see if that makes a difference.
The next thing I would suspect is malware. Possibly caused by P2P, torrents, toolbars, search programs etc.
Next try How to Reset or Restore Advanced Settings in Internet Explorer 7 I say this because many add-ons or malware add-ons can cause issues that this might remove.
If that works you should consider running our malware removal proccess.
To be quite honest it does sound like you have some programs and/or malware that is causing this. Remove these items if you have them.