ERROR: Unable to access target machine for request: "IUSH4YZ5", machine name: "SNDSRV01", error code: 121

This is the error code I getting when trying to install the client on certain W2K domain controllers. I'm running SMS 2.0 SP2.

An error 121 indicates an access token problem. Here is what I can tell you about NT clients

On a workstation
When a Site Server targets a workstation prior to pushing the SMS client software, it writes an "access token" entry in the "SMS" Software Key of the target's registry. This is to lock the workstation from being targeted by other Site Servers during SMS software installation processing. The Error 121 event occurs when the Site Server cannot read back the access token it just wrote. This was a bug that was supposed to be fixed in SP1 but I experienced over a hundred cases of this particular error during our rollout this past summer.

The workaround is to delete the access token subkey and reboot the machine. When the DDR in the "CCRetry.box" on the Site Server is processed (this can be expedited by dropping it in the CCR.box manually), the client installation will continue to completion.

On a Domain Controller
DC clients share a lock that is stored in the registry of the Domain's PDC. When installing the client on itself, a Site Server will first write it's NetBIOS name in the PDC's registry. The problem comes about when multiple SMS DC's clients are attempting to install in the same time period (for example we were doing 50 Primary DC sites each Friday night towards the latter part of our rollout). This is because the replication time for the new SMS Domain Account back to the DC can take up to an hour (or more - look for those synch messages in the "CCIM32.LOG" on the clients). When a DC SMS Site Server cannot puts its name in the PDC's registry, because it is held by another DC SMS Site Server, an Error 121 results.

Now how you can apply this to Windows 2000 Active Directory domain structures is for you to inform me, mainly because I don't have the environment. However, the first thing I would look at would be 271724 "SMS May Time Out Attempting a PDC/BDC Resynchronisation in a Large Windows 2000 Active Directory environment" which speaks directly to SMS client failures on W2K Domain Controllers due to the still present resynchronisation issue.