Error when I startup Vista...

Well..My sister has an error on her laptop, and asked me if I knew how to fix it.. I tried system restore, but it didn't work..And I am not so familiar with Vista. When I start up her laptop, this error comes up

Title - LogonUI.exe - Bad Image

Code:

C:\Windows\System\shacct.dll is either not designed to run on Windows or it contains an error. Try installing the program again using the original installation media or contact your system administrator or the software vendor for support.

Does anyone know how I can fix this error?