First, do a hard shutdown by depressing and holding in the Start button for 5 seconds. Then restart. See whether the problem is resolved.
If the problem is unresolved, do a 'System Restore' by going into Programs, Accessories, System Tools, and then select System Restore. Follow the prompts.
If the problem is still unresolved, do a repair install of XP.
If none of these things work, do a complete backup of all data, and then do a re-installation of XP. You will have to re-install all programs.