I have not been able to reproduce this issue here so far. I tested mIRC under Windows XP SP3 with DEP enabled for all programs and under Windows 7 which shows mIRC is running under DEP. Please try the following:

1. Install mIRC to a clean, empty folder in your my documents folder.
2. Open that folder and right-click on the mirc.exe file.
3. Select "Create Shortcut".
4. Right-click on the newly created shortcut.
5. Select "Properties".
6. In the "Target" location, where you see "Some\Path\mirc.exe" change that to "Some\Path\mirc.exe -rtest"
7. Click OK.

Now double-click the shortcut icon and it will run mIRC and use the "test" folder for all settings. This should ensure that no other settings on your system are used. Does that work?