Wallpaper and desktop theme are corrupted….

Hello all,
I am experiencing a problem with my desktop computer (Dell, Inspirion 4500, Win XP home edition).
A new wallpaper is “stuck” on my desktop and I cannot change it any more. When I restart my PC, the old wallpaper shows for a sec, then a new simple blue wallpaper takes over.
When I try to modify it (display properties/ desktop) it is frozen. I cannot browse, or select a different pic. I tried every method I can think of, but nothing helps.
Q – Is there a way to reinstall only the desktop theme/ wallpaper pic, without performing total windows reinstall?
Thanks for your help,
Mike