Deactivate DEP on Win XP?

I seem to be having a DEP revolt on my laptops!! I have 3 laptops that all have a DEP error on start up. The message says that DEP has closed win explorer and hence nothing on the desktop is accessible. i cannot run any programs except via task manager. Please help, how do i get DEP to allow explorer.exe to run so that I can even run some virus diagnostics?