To disable the DEP, you reboot the computer and hit F8 to enter the boot selection menu. Select Safe Mode. Once the computer is booted up in Safe mode, click on Start and Run and type CMD and hit enter Type the following;
bootcfg /raw "/noexecute=alwaysoff /fastdetect" /id 1
See the following article for more information: http://support.microsoft.com/kb/886348