rxvt: command not found

Have You tried accessing the binary with the full path?

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/rxvt - from package: rxvt

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