Fault with iTunes 8 and QuickTime - Network resource unavailable

Question

This is a similar query to one already found in the iTunes discussion pages however I could not see an answer there as to how to fix this without downgrading so I have posted here. I do not wish to downgrade, I would much rather get iTunes 8 working properly.
My problem is as follows:
I downloaded iTunes 8 (via Apple update) and once I had done so it both iTunes and QuickTime worked fine on my pc. However, after I turned my pc on again the next time (I have Windows XP Home Edition and Service Pack 3) and clicked the iTunes shortcut, I got the following message:
"This action is only valid for shortcuts that are currently installed"
I tried re-downloading and re-installing iTunes 8 and QuickTime (both manually and using Apple Update) several times and each time I got the following error:
"The feature you are trying to use is on a network resource that is unavailable. Click OK to try again, or enter an alternate path to a folder containing the installation package QuickTime.msi (or iTunes.msi) in the box below."
I found the locations of the appropriate .msi files, selected OK, but was given another message:
"The Path C:\Documents and Settings\username\etc cannot be found. Verify that you have access to this location and try again, or try to find the installation package QuickTime.msi (or iTunes.msi) in a folder from which you can install the product QuickTime (or iTunes)."
This same thing happened when I tried to uninstall each program.
At the moment, I have no functioning iTunes. I have also tried restoring my system but due to another unrelated issue with my OS, this did not work properly and I am still in the same position.
Any help that anyone can provide would be most gratefully received.
Thank you.

Answer 1

It sounds as though things have got in a bit of a muddle. Usually removing iTunes and related programs and doing and installer file clean up fixes this:
Download a fresh copy of iTunes and the stand alone version of Quicktime (the one without iTunes)
http://www.apple.com/quicktime/download/win.html
http://www.apple.com/itunes/download/
Download and install Microsoft Installer cleanup utility, there are instructions on the page as well as the download. Note that what you download is the installer not the program – you have to run it to install the program –All Programs>>Windows Install Cleanup
http://support.microsoft.com/kb/290301/
Now use the following method to remove iTunes and its components:
XP
http://support.apple.com/kb/HT1925
Vista
http://support.apple.com/kb/HT1923
If you hit a problem with one of the uninstalls don't worry, carry on with the deleting of files and folders as directed in the method.
When you get to deleting Quicktime files in the system32 folder as advised in the method, you can delete and file or folder called Quicktime.
Restart your PC.
Run the Microsoft Installer Cleanup Utility. (Start > All Programs > Windows Install Clean Up)
Remove any references you find to the programs you removed - strictly speaking you only need to worry about those programs where the uninstall failed.
restart your PC
Install the stand alone Quicktime and check that it works.
If it does, install iTunes.