Data Execution Prevention error message

Within the last couple of weeks i have had a couple of users call me and tell
me that they have received an error box about the DEP. I have also experience
it on my system and really confused as to why we are getting these messages.
Nothing has changed on the computers.. no new software (except for ms
updates) or even hardware. the systems that these machines are getting the
DEP message are Windows XP SP3… the users who are running w2k have not called me on this so i don’t think they are having this issue just those who are running XP SP3.

What can i do to stop this besides turning DEP off?