Select PAE Mode option when the target device’s operating system is enabled with PAE (such as Windows Vista), and the Cache Type in the vDisk properties is set to Cache in device RAM.

Citrix Provisioning Server 5.0 (SP1, SP1a) Administrator’s Guide
http://support.citrix.com/article/CTX117916
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その他、参考情報です。
PAE
http://msdn.microsoft.com/en-us/library/ms791485.aspx
BCDEdit /set
http://msdn.microsoft.com/en-us/library/aa906211.aspx