In our vSphere 5.1 update 2 environment we found that some of our Windows virtual machines were running guest customization every time they boot. This was causing them to loose their static IP address and take an additional 5 minutes to boot.
To fix this issue, boot to Windows and open regedit. Navigate to to the following location:
Edit the BootExecute key and remove all sysprepDecrypter.exe lines. We had some VMs with up to 10 entries!
Your key should now look like this:
Click OK. Now the VM will not perform the guest customization on every boot.
To prevent future VMs you deploy from gaining this issue update your vSphere environment to at least 5.1 Update 5.