2 more things to try:
First, make sure you have XP Service pak 1 (and any other critical updates) downloaded from http://www.windowsupdate.com
Then... go to http://security.kolla.de and download the free 'SpyBot Search and Destroy' adware&scumware cleaner.
After you install it, run it, go to the 'online' tab, select any updates available and install them.
Then... go to 'settings' and select the 'settings' sub menu.
Scroll down the list and find the check box under 'web update' and select 'Display also available beta versions'
Go back to 'online' and scan for updates again. Install them if any are found.
Next click on 'check for problems' on the lower left.
Let this scan your whole system. When completed, delete all items that are listed in RED.
Optionally you can run the 'immunization' tool afterwards.
I have had some success with misbehaving systems (where they couldnt even open an Excel file!) by using this program in the past. It may not fix your Quicktime problem. But it certainly wont hurt.
