Yes, there is an option that disables the symbol browser. It is a checkbox at: Settings > Editor > "Code completion and Symbol Browser" > "Symbol Browser" tab. If you see the "Disable symbols browser" option unchecked, though, something else is wrong.
Did you install version 10.05 on top of an older version?
Perhaps migrated settings could have caused this odd behaviour. To be sure of this, I would go to your %AppData% folder (I don't remember where it was in XP, but I think this variable exists) and manually delete the codeblocks folder there. This will erase all settings, and the next time you run Code::Blocks, it will be setting itself up as if it was running for the very first time.