Hi Ian,
In Exchange 5.5, to determine whether you are running Enterprise or Standard Edition, you would look for certain Informational Events in your Application Log. For example these Events will be logged in Exchange 5.5 when your Information Store is started:
Event ID: 1217
Source: MSExchangeIS Private
Type: Information
Category: General
Description: Information Store with unlimited storage capacity enabled
If you are logging the Event ID 1217 then you are on Enterprise Edition. You will log an Event ID 1216 indicating "limited storage capacity" if you are running Standard Edition.
In Exchange 2000, the Event IDs are the same but the verbage is slightly varied.
Hope this helps
Melissa