error when using PHP with IIS 5

I get the following error when trying to test PHP with IIS 5 using tha Isapi DLL. Any ideas what is causing it? The HTTP server encountered an unhandled exception while processing the ISAPI Application ' php4isapi!HttpExtensionProc + 0x44 wam!SE_TABLE::ReleaseExtension(class HSE_BASE *) + 0x99A wam!SE_TABLE::ReleaseExtension(class HSE_BASE *) + 0x30A w3svc!IIS_VROOT_TABLE::AddRefRecord(void const *,int) + 0x2E4 w3svc!PARAM_LIST::CanonList(void) + 0x242E w3svc!HTTP_REQ_BASE::BuildURLMovedResponse(class BUFFER *,class STR *,unsigned long,int) + 0x3979 w3svc!HTTP_REQ_BASE::BuildURLMovedResponse(class BUFFER *,class STR *,unsigned long,int) + 0x3915 w3svc!HTTP_REQ_BASE::TestConnection(void) + 0xDE5 w3svc!HTTP_REQUEST::ReprocessURL(char *,enum HTTP_VERB) + 0x25A w3svc!DumpW3InfoToHTML + 0xA95 w3svc!DumpW3InfoToHTML + 0x1813 w3svc!HTTP_REQ_BASE::TestConnection(void) + 0xE5C w3svc!ScanForTerminator(char const *) + 0x548 w3svc!IIS_SERVER_INSTANCE::QueryMaxEndpointConnections(void)const + 0x331E w3svc!IIS_SERVER_INSTANCE::QueryMaxEndpointConnections(void)const + 0x323D ISATQ!_AtqGetCapTraceInfo@4 + 0x644 ISATQ!_AtqGetCapTraceInfo@4 + 0x934 + 0x4CE0377A '.