Replace xscreensaver With gnome-screensaver (Xubuntu)

I have recently apt-get remove'd xscreensaver and apt-get install'd gnome-screensaver. This appears not to have worked because when I press the Lock Screen menu item, nothing happens. I have tried putting gnome-screensaver in the list of autostart applications but this doesn't help (must it be started before anyone logs in?). How can I make gnome-screensaver work properly and make the Lock Screen button do what it's supposed to? Thanks in advance, ell.

EDIT: I am using Xubuntu 11.04 and the output of ps aux | grep saver is:

elliot    1374  0.0  0.3  27344  6024 ?        Ss   19:35   0:00 gnome-screensaver
elliot    1669  0.0  0.0   4160   848 pts/0    S+   19:48   0:00 grep --color=auto saver

But I think the above information will now be irrelevant as it seems that locking the screen now works, when I lock the screen I get a black screen and when I move the mouse, the unlock dialog pops up so now all I need to do is set a screensaver, can anyone help me out on that?

Installing

When you have removed xscreensaver and installed gnome-screensaver you will need to logout and login for the XFCE lock-screen menu item to work

Setting your screen saver

Installing gnome-screensaver also installs gnome-screensaver-preferences.

If you run this in a terminal you will see this screen which will allow you to change from the "black screen" to one of the gnome-screensavers:

Integrating with Settings Manager

When installing gnome-screensaver it will create a file called /usr/share/applications/gnome-screensaver-preferences.desktop

To integrate this into Settings Manager then replace the contents of this file with the following:

[Desktop Entry]
Name=Screensaver
Comment=Set your screensaver preferences
Icon=preferences-desktop-screensaver
Exec=gnome-screensaver-preferences
Terminal=false
Type=Application
Categories=Settings;DesktopSettings;Security;X-XFCE
NotShowIn=KDE;GNOME;

You don't need to remove Xscreensaver. After you install gnome-screensaver, you can change the file /usr/bin/xflock4 to give priority to grome-screensaver.

Install gnome-screensaver:

sudo apt-get install gnome-screensaver

Open file:

sudo nano /usr/bin/xflock4

Find

# Lock by xscreensaver or gnome-screensaver, if a respective daemon is running
for lock_cmd in \
"xscreensaver-command -lock" \
"gnome-screensaver-command --lock"

Replace

for lock_cmd in \
"gnome-screensaver-command --lock" \
"xscreensaver-command -lock"

May need logging out and in again.