Cannot connect to some Domain computers

We have some XPProSP2, on a 2003 Domain that we cannot connect remotely to,
and, trying to run Manage from another computer fails. We often get error
messages like "not a manageable OS". We use DameWare, and, IF the agent was
already installed on the computer, we can get into it. Software/patch
deployment from SCCM or Shavlik fails, too.

However, when ON the computer, all seems fine. Users can logon, both
locally and to the Domain, and, all seems well.

I presume that this is some network or firewall issue (maybe a rights issue,
but, then it'd fail locally), but, am unsure on how to troubleshoot.

Any ideas/tips would be appreciated!

Thanks