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If this fails, then try again, but leave out everything but your bookmarks -- The more files you restore, the greater the chance that one of those files is corrupt and is the cause of your problem.
On Windows XP, Firefox profile directories are typically located in
C:\Documents and Settings\<account-name>\Application Data\Mozilla\Firefox\Profiles\
and the cache files may be in
C:\Documents and Settings\<account-name>\Local Settings\Application Data\Mozilla\Firefox\Profiles
Also delete the Firefox directory from Program Files as well -- There may be some leftovers in there even after "uninstalling."
I suggest you delete everything you don't absolutely need to keep. This includes minimizing 3rd-party extensions and themes; Every one you add is a potential reliability risk, so convenience and 'cool factor' must be balanced with reliability.
Jim