Well, to start things off, I was having problems with the game Unreal Tournament 3 receiving a "stopped working" crash. I figured out it was Data Execution Prevention that was shutting the program down. After finding out that I could not add ut3.exe to the DEP exception list,

(control panel/system and maintainence/advanced system settings/ performance settings/ Data Execution Prevention)

,I decided to turn it off by typing this into the cmd and restarting.

bcdedit.exe /set {current} nx AlwaysOff

This worked, but after UT3.exe crashed again, this time with a "memory could not be read", I figured that DEP was actually turning it off for a good reason, so I turned it on again.

bcdedit.exe /set {current} nx AlwaysOn

That worked too, but now for some reason, the options in the DEP section in the control panel are now greyed out. Also, it was at this time that I began getting "stopped working" errors with not just UT3 but suddenly numerous other games as well. I figured it was something to do with the greyed out properties in the control panel. DEP was definitely running differently, so I tried taking out the {current} in the cmd command.

bcdedit.exe /set nx AlwaysOn

I'm not sure if this did anything. Anyway, I have no clue how to get DEP back to the state it was in before this fiasco, so could someone help me out?

1. To check you system files, recommend you go to the Accessories
Right click on Command Prompt
Click on Run as administrator
enter: sfc /scannow
There is a space between "c" and "/".
This could take from minutes to hours to run.

If this command doesn't run to 100%, then you probably have a bad hard drive sector.

Are the greyouts now selectable?

2. Do you want DEP on or off?

3. You might check you firewall. Are the games listed as exceptions?

Vista and DEP tend to not like games, particularly Half-Life based games.

If you manage to stop it being greyed out I'd advise you to turn off DEP, or at least only use it for only for essential MS programs.