You should consider installing SP1 as soon as possible

ISA Server 2004 SP1 can be applied to ISA Server 2004 Standard and includes:

• All software 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.

Download SP1 for ISA 2004 (6.7mb)

See the Readme file for SP1

Required Patches

Internet Security and Acceleration (ISA) Server 2004 with SP1 running on a Windows 2000 SP4 Server box does not currently require any security-related patches.