According to this url http://ms-abhay.blogspot.com.au/2010/08/could-not-load-dll-xpstar90dll-or-one.html, you need to reinstall the SQL Native Client. It is unusual for it to become corrupt, but since there are a few cases of it occuring around the net, I'd guess it must be a combination of rare factors. Install SP4 for SQL2005 and see how you go.