Rather then looking to see if the user has changed your screensaver, why not just remove the option in the control panel. All you have to do is set on registry key, and users will have to manually edit the system.ini in order to make any changes. The registry key to edit (or add if missing) is:

Path:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System
Key:
NoDispScrSavPage
Value:
REG_DWORD
0=disabled (screensaver option is in the control panel)
1=enabled (screensaver option is removed from control panel)

I know this works for NT/Win2000, but I have never tried it on Win95/98.