You can install exchange 5.5 with no problem on a windows 2000 server. Except that what I have noticed exch 5.5 doesn´t like w2k sp4 that much....
and you should install the latest exch 5.5 sp... is sp4 I beleive..
Petje
A+, MCP, MCSE on NT4.0 and windows 2000 with messaging specialty and CCEA