Was this an upgrade from a previous version of Ubuntu? And if so, what version?
Are you able to install software from Synaptic Package Manager, or using apt-get from the terminal?
What is the output of:
sudo apt-get install gkrellm
Gkrellm is a light weight system monitor that I use all the time.
Also, can you post your /etc/apt/sources.list file.
And welcome to the LQ forums!