I recently had to turn off DEP on my Vista PC (see my post in
microsoft.public.pocketpc.activesync for the reason). To turn off DEP I
followed this advice:

John Barnett's Windows Vista Support: Disable Data Execution Prevention (DEP)

Once the problem with Windows Mobile DEvice Center had been resolved, I
turned DEP back on using the same advice as above.

Unfortunately turning it back on with this advice meant that DEP prevented
Internet Explorer from opening!

I tried to use Control Panel/System/Advanced Settings/Performance
Settings/DEP to switch DEP off for IE, but the tab is all greyed out with a
little "No Entry" sign and I cannot select any of the bullet options.

How can I restore DEP settings to defaults?