session_test.php:
<?php
session_start();
session_register("SESSION");
if (! isset($SESSION)) {
  $SESSION["count"] = 0;
  echo "<li>Counter initialized, please reload this page to see it increment";
} else {
  echo "<li>Waking up session $PHPSESSID";
  $SESSION["count"]++;
}
echo "<li>The counter is now $SESSION[count] ";
?>

Using IIS4 & the precompiled PHP 4.0 binaries dated May 22 2000.
Using the isapi dll
Event viewer error message:

The HTTP server encountered an unhandled exception while processing the ISAPI Application '
php4isapi + 0x1127
wam!SE_TABLE::ReleaseExtension(class HSE_BASE *) + 0x526
wam!SE_TABLE::ReleaseExtension(class HSE_BASE *) + 0x1313
wam!SE_TABLE::ReleaseExtension(class HSE_BASE *) + 0x11A8
w3svc!IIS_SERVER_INSTANCE::QueryServerSize(void)const  + 0xC8
w3svc!PARAM_LIST::CanonList(void) + 0x2BE8
w3svc!HTTP_REQ_BASE::BuildURLMovedResponse(class BUFFER *,class STR *,unsigned long,int) + 0x377E
w3svc!HTTP_REQ_BASE::BuildURLMovedResponse(class BUFFER *,class STR *,unsigned long,int) + 0x36EE
w3svc!HTTP_REQ_BASE::TestConnection(void) + 0xA42
w3svc!ScanForTerminator(char const *) + 0x386
w3svc!HTTP_REQ_BASE::CheckForBasicAuthenticationHeader(char *) + 0x30FC
w3svc!HTTP_REQ_BASE::CheckForBasicAuthenticationHeader(char *) + 0x3094
ISATQ!_AtqPostCompletionStatus@8 + 0x645
ISATQ!_AtqPostCompletionStatus@8 + 0x862
KERNEL32!lstrcmpiW + 0xBE
'. 

Hope this helps.