That could have many reasons. Check the following steps:
If the CD is damaged, the system might be unable to read certain data at some point. Also, your CD drive could be faulty. You can try installing Ubuntu via USB stick (tutorial for Windows here and for Ubuntu here) to see if it's a problem with the CD (drive).
- How fast is your CPU? How much RAM do you have?
I personally experienced unexpectedly long waiting times when installing Ubuntu 12.04 or later on systems with poor hardware - especially on netbooks. I don't know how much time I waited, but I think one time I was waiting for about 10 minutes.^^ Don't worry, the operating system won't be so slow afterwards.