Why am I getting these messages?

"Microsoft Excel has encountered a problem and needs to close. We are sorry for the inconvenience"
"Insufficient memory"
"Not Enough system resources to display completely".
"Insufficient system resources exist to complete the requested service"
“CONTACTING server FOR INFORMATION"
“Excel cannot complete this task with available resources. Choose less data or close other applications”
"Selection too large"

Also, processing (calculating, saving, shifting cells, etc) is extremely slow.