try uninstalling Firefox, delete your profile folder, then install Firefox again. Your profile folder on XP is
C:\Documents and Settings\"whatever the user name is"\Application Data\Mozilla
deleting you profile is very important to get a "fresh start" on firefox. if u dont delete your profile then it will go back to the way it was before.
You will want to save your bookmarks before you do this. You can export them to a html file with the bookmarks manager. Just go to "Bookmarks", "Manage Bookmarks", "File" then "export"
Then when you are done reinstalling Firefox import your bookmarks back the same way. This usually clears up any problems that continue even after you uninstall and reinstall Firefox. You will have to reinstall your extensions if you have some installed.
i hope it works
edit......
if u cant find "application data" then do the following:
1. Start
2. Control Panel
3. Folder Options
4. View
5. Show hidden files and folders
then try to find it again. it should be there. again...i hope this works for you