xscreensaver-demo: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/xscreensaver-demo - from package: xscreensaver-base

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/xscreensaver-demo - from package: xscreensaver

You may install the required package with command apt-get install {package}

Error in other languages:
xscreensaver-demo: bevel nie gevind nie
xscreensaver-demo: comandă negăsită
xscreensaver-demo: comando não encontrado
xscreensaver-demo: commande introuvable
xscreensaver-demo: command not found
xscreensaver-demo: käsku ei ole
xscreensaver-demo: không tìm thấy lệnh
xscreensaver-demo: komanda nerasta
xscreensaver-demo: Kommando nicht gefunden.
xscreensaver-demo: kommandot finns inte
xscreensaver-demo: komut yok
xscreensaver-demo: nie znaleziono polecenia
xscreensaver-demo: níor aimsíodh an t-ordú
xscreensaver-demo: no se encontró la orden
xscreensaver-demo: no s'ha trobat l'ordre
xscreensaver-demo: opdracht niet gevonden
xscreensaver-demo: parancs nem található
xscreensaver-demo: perintah tidak ditemukan
xscreensaver-demo: príkaz nenájdený
xscreensaver-demo: příkaz nenalezen
xscreensaver-demo: команда не найдена
xscreensaver-demo: командата не е открита
xscreensaver-demo: 命令找不到
xscreensaver-demo: コマンドが見つかりません