I am also getting this exact same error since the new b394a was released. It occurs on the start up of the program. I am also running Windows XP Pro x64. I have had no problems when running OSU in the past, but this last update broke it for me.
Issue was resolved by reloading all of Osu. I just downloaded the installer and re-inserted my songs. Something about the updater messed with a file in the wrong way for me.