If you are installing as a user with administrative rights on a CLEAN machine (a machine which never had a version older than 5.0.81 on it) you should not get this message.
If this is that case and you are still getting this message I assume it's because you are installing an older version even though you think you are not. You should be doing a network install from your licenseadmin server by browsing to the TCP5_USER share on the machine where you setup the licenseadmin server and running setup.exe (remember the TCP5_USER share MUST be full control permissions for ALL users). That machine with the licenseadmin server should have the current version installed on it. If it does not then you will not be able to successfully get a sequence number. In other words, if you just try to install the update on your win2k PC and the licenseadmin server still has a version older than 5.0.81 on it you will not be able to get a sequence number. If this is the case, update your licenseadmin server before you do a network install. See other posts for more details on updating the licenseadmin server.