You should consider installing SP2 as soon as possible
ISA Server SP2 can be applied to ISA Server Standard and Enterprise Editions, and includes:
All hot fixes and security updates issued since ISA Server was released to manufacturing.
Fixes for common issues reported by customers through Microsoft Product Support Services (PSS).
Enhanced stability of the ISA Server services and administration tool in a number of scenarios.
Fixes that enable ISA Server to run on Microsoft Windows Server 2003, Standard Edition and Windows Server 2003, Enterprise Edition.
Fixes recommended through an audit by third-party security experts.
Download SP2 for ISA 2000 (3.9mb)
See the Readme file for SP2
Internet Security and Acceleration (ISA) Server with SP1 running on a Windows 2000 SP4 Server box requires the following security-related patches: