I have been getting this error:

Can't open file 'c:/windows.old/program files (x86)/world of warcraft main/interface/addons/failbot/failbot.toc' (error 5: access is denied.)

repeted twice in Details <<

Then I get the windows error message

"CurseClient.exe has encountered a problem and needs to close. We are sorry for the inconvenience"

I tried uninstalling the client, with removing my settings.

I am running windows Xp

If you are running Windows XP, why do you have \windows.old\program files (x86)\.... ? That directory structure is only created if you install Vista or Windows7 beta on top of Win XP. Are you sure that path and directory exist, or that is where your WoW is installed? For that matter, why do you have "main" in the path name?

Windows XP install path is c:\program files\world of warcraft\interface\addons (caps where applicable). The only other reason you might have (x86) is because you have Windows XP 64-bit, in which that is OK.

Thats what it looks like to me that he either has xp64.  Since there is a window.old directory looks like he reinstalled windows without doing a clean install.  I would think that there is more to this that the OP needs to let us know bucause I'm just speculating. Now if all that were to happen most likely the user that your are logged in as doesn't have enought permission to access that folder so neither does the cures client.  You could try running the CC as admin and see if that works.

Look at this article http://support.microsoft.com/kb/305780 and see if it help but we probably need some for info.