Great tip from Tony Campbells book reproduced here
http://vista.beyondthemanual.com/2006/11/vista_tip_turn_off_data_execut.html
Data Execution Prevention (DEP) is a security feature introduced in Windows XP, Windows Server 2003, and now in Windows Vista, that looks for malicious code trying to execute. If DEP's analysis of a process beginning execution makes DEP think the resulting code will cause some sort of unwanted activity, DEP intervenes and shuts the process down. It sounds good in theory, but too often DEP shuts down legitimate programs--particularly third-party installers used by software developers that release their products for download off the Web. Equally too often, DEP fails to show any sort of warning or information prompt telling you it shut off a process, leaving you scratching your head, wondering why your machine is ignoring you. You can turn DEP off on a per-application basis, but there's a quick way to simply disable the feature entirely.
If you have adequate defenses in place, DEP won't really provide you any benefit and will likely get in your way.
Why not turn it off? Turning DEP off completely requires venturing into the command prompt space.
Open the Start menu, and in the search box, type "Cmd" and wait for the results box to populate. Right-click on the cmd shortcut that appears and select Run as administrator.
You'll probably have to acknowledge a UAC prompt to get it done. Once you're at the command prompt, type the following:
bcdedit.exe /set {current} nx AlwaysOff
You should get a success message back. If you ever want to turn it back on, use the same procedure to get to an elevated command prompt, and then type:
bcdedit.exe /set {current} nx AlwaysOn