We are using VMWare ESXi 3.5, and have found it quite useful for cloning a live server to then use the virtualized version to test/practice software upgrades.
The trouble is, when the virtualized version fires up, it registers itself on our domain (Active Directory), causing the original server to no longer be accessible via Windows shares. The fix is to remove the virtualized version from the domain, configuring it to use a workgroup instead, deleting the Computer account in AD, and then removing the real server from the domain and re-adding it.
Is there a better procedure?
Note, we cannot simply disconnect the virtual network from the virtualized server, as it needs to be connected to the network to actually be removed from the domain.
To avoid a lot of headaches, always run sysprep on a copied Windows image. Create an answer file to configure the PC when it first starts up, including auto-joining the domain.
My recommendation would be to create a second VMware network on the VMware host that's isolated from the production network and configure the converted server to connect to this network. That keeps your domain intact, negating the need to perform the steps you're currently performing, which in my opinion are slightly wacky (no offense intended). If you need the converted server to have access to domain resources (DC, file, print, etc.) for the purpose of these tests then convert those resources as well and configure them to use the second network as well. Getting these converted guests access to the internet if needed should be a simple matter of configuring your router and/or firewall to recognize the second network (VLAN, NAT, etc.).
The key is to keep the second network physically and logically isolated from the production network.