http://www.sibelius.com/cgi-bin/helpcenter/search.pl?program=1&version=Any&platform=2&words=a563&topicid=0&com=search&days=0&language=1&highlight=1&type=natural&search=Search

Article 563 on Sibelius KB explains some fixes for this.
Also other possible solution is to ensure that if you have Microsoft JIT debugger installed, under your Add/Remove Programs list, that you click change. This should fix the installed version of JIT debugger.