First i tried the Technote:
http://www.symantec.com/business/support/index?page=content&id=TECH140545
The File could be get from here instead of asking the support (took me an hour in the support hotline)
http://rapidshare.com/files/445627992/Altiris_Agent_Install_7_0_7437.zip
But after that the Problem was still not fixed. It now tooks less CPU Time but still 6 - 10% CPU isn't quit nice.
Then i check the logs and found ot that it should had somthing to do with the policies.
And we had two Policies that had the checkbox „User must turn on from Altiris Agent“ under the schedule settings enabled.
After disable this setting and updating the client, the AeXNSagent Task went down to nearly 0%
So maybe check if one of the affected computers have any package with this setting and disable it for testing.