Please give us more information.
What operating system are you using? Which processor do you have and how much RAM? How much free, clean, freshly defragmented space is on your hard drive?
If you are using a Windows machine, have you gone to Windows Update and ensured you are getting Microsoft Updates as well as default Windows Updates?
Have you got the latest version of Quicktime from Apple, per the program's requirements?
Are you installing from a disc or from a download? Have you ensured you have the correct (32-bit or 64-bit) version of the program for your operating system?