I Need Help Installing Itunes On My Computer

everytime I try to download it, the installation process starts off fine, but then in the middle of installing it stops and say "Installation of Quicktime did not complete successfully. Itunes requires quicktime" then it says "the installer encountered errors before itunes could be configured, your system has not been modified" I already have quicktime downloaded onto my computer so I dont understand why computer keeps saying this..............................can you help me??????