logically i assume the computer is joined to the domain already
but i have to ask: the computer is actually joined to the domain and working good with it?
for another way:
you can create an MSI package to deploy the avast clients too, if you cant get it to work then i would suggest installing the client with an MSI package (GPO deploy?) and dont put too much effort anymore in getting it remotely deployed since there should be a new ADNM version on the way with the avast 5 setup (but from what i heard i could take up to 6 months before that product is here, but thats hear say....)
i also see that you use the account administrator... have you tried with a different account that also has installation rights?
You sure that you filled in the accounts username in the adnm as: domain\username if you used without the domain in it, avast will deploy using the standard computer account. If you have set your local administrator account password the same as the domain password then its logical that in XP the installation succeeds since the local admin account is enabled, where in Windows 7 this account is disabled by default, therefor possibly creating an error like you have