ISA Server 2000 settings and features that are not supported when you migrate to ISA Server 2004: http://support.microsoft.com/default.aspx?scid=kb;en-us;840697
The migration stops responding when you try to migrate configuration settings from an ISA Server 2000–based computer that uses SSL certificates to an ISA Server 2004–based computer: http://support.microsoft.com/default.aspx?scid=kb;en-us;888643