So, I downloaded the latest version. I logged in to my administrator account, installed Quicktime, and played a quick video to verify that it was working. When I logged out and logged back in to my regular limited-user account, Quicktime no longer functioned in this user account. At all. Gives an error message saying "Error 46: Could not find or load activex control". Funny, the previous version worked just fine.
Further investigation revealed it was a problem with some DLL's not being automatically registered for this user. Now, as you may know, you can register DLLs manually using a console command, regsvr32
Here is what I ended up having to do to fix it:
1) Log in as administrator, and temporarily set the limited user account to another admin account.
2) Switch to User account, install Quicktime.
3) Switch to true Admin account, start running a registry monitor program, and attempt to manually register the first DLL.
4) When that fails, examine the registry monitor trace to find where regsvr32 tried to change a registry setting with an "Access Denied" result, to determine what registry key needs to be changed.
5) Switch back to temporarily-admin User account, open the registry editor, find the registry key from step 4. Edit the registry key's permission settings, adding the Administrators group with full read/write permissions.
6) Switch back to the true Admin account. Repeat steps 3-6 until the DLL registers successfully (there may be several registry keys that need to have their permissions changed before it will work).
7) Repeat steps 3-7 for each of the 4 DLL's that need to be registered.
8) Reset the User account back to a limited-user account type.
Here's the problem: Apple seems to have set up their Quicktime installer under the assumption that you, the end user, use a single account for everything you do on your Windows PC. But Windows XP, especially the recent service packs, encourages you to set up a limited-user account for everyday use, and stay out of the administrator account unless truly necessary. This has always been the way serious geeks behaved, and XP now encourages regular folk to adopt the same habits. In this environment, in this day and age, why would Apple apparently not even bother to test Quicktime installation in such a system configuration? It's just baffling.
Now, it's also possible that there was something peculiar to my system that caused the problem. But since the root of the problem was a permissions issue with some registry keys specific to Quicktime, which means the registry keys themselves must have been created by the Quicktime installer, I can't imagine what there could be about my system that would be so different than what they developed/tested the installer on. The registry settings may have been created by the installer for a previous version, but surely they would have tested installing the new version over an existing install, right?