In ConfigMgr 2007 you may find that when you manually or programmatically remove a machine that has been either decommissioned or reimaged from the “All Systems” collection that resource name reappears in the collection(s) during the subsequent Active Directory Systems Discovery process.
This functionality is by default because ConfigMgr has determined that the NetBIOS name is still an active member of the domain(s) you have configured as a ConfigMgr domain(s) even though to your knowledge the machine is a nonexistent resource.
Active Directory Systems Discovery determines active computer resources by looking for machine or computer accounts in the Active Directory Domain Services. When it finds a new potential client resource it attempts to ping the computer and if a ping request is confirmed as having a successful IP Address Name Resolution a Data Discovery Record (DDR) is created for the computer resource and it is “Discovered” and this is when the machine reappears in your ConfigMgr database.
To combat this issue you must remove the computer name(s) from the Active Directory Users And Computers application. Then you can remove the resource once again from your site server. After this has been accomplished the resource should not reappear during the next Active Directory Systems Discovery process interval
Note: You should also enable and configure DNS Scavenging on your DNS servers as well to ensure that the issue does not reemerge. For additional information see the post below:
DNS Aging And Scavenging
http://myitforum.com/cs2/blogs/dhite/archive/2007/09/09/dns-aging-and-scavenging.aspx
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