Exchange 5.5 SP3 or higher runs perfectly on W2K
But keep in mind that it needs a DC, so your W2K box HAS to be and AD domain controller(!)
Here's the scenario (brief description):
1. Install Exchange 5.5 onto the W2K server, incl. Service Packs. Make sure the Site and Organization names match the ones on the old server, but do NOT join that Organization.
Also, make sure the DB and logfiles are on the same driveletters as on the old server (you can always change it afterwards)
2. shut down exchange on the old server
3. copy the DB files + logfiles from the old server to the new server
4. run isinteg -path, AFTER you tried to start the Information Store
5. Synchronize de DS with the IS (Consistency Checker in de Exchange Admin)
6. Watch out: your Primary Windows NT accounts will be empty, use the export/import tool to correct that.
There's also a second scenario:
when installing 5.5 on the new box, join the Organization of the old server. Then, by using the Exchange Admin, you can move the mailboxes to the new server. But this is a rather slow process, so be aware
Hope this helps