So are you installing ITunes, and Quicktime cannot be installed ?
It is unclear from your statement, whether the ITunes install
is failing or the Quicktime install is failing.

Quicktime is used as a helper for ITunes, so the two are used
together. And being an essential helper, the ITunes installer
has to be prepared for situations where the customer doesn't
have Quicktime on the computer already.

If the context and symptoms of your problem are different, you
should start a new thread, describing exactly what is going on.
Complete error messages and everything. Even if you don't understand
what is going on, the complete error messages may suggest to
someone answering your question, what is broken.

Something like:

1) I tried to install ITunes 8.0.2.20.
2) The ITunes part seemed to be going good.
3) Then it said it was going to install Quicktime.
I already have an older version of Quicktime 4.3.2.1 on my computer.
4) The Quicktime installer said "the older version of quicktime
cannot be removed" and it quit.

If you can provide some detail, you'll get better answers.

Paul