Hi Val. Sorry about the delay. My internet connection went down in the middle of trying to reply. (Bloody Wellington.)
Okay ... if it is Windows Installer, then it's unlikely to be the Windows installer service
itself, because it's being too picky about not liking apple products. So that suggests either some issue with the Apple program .msi files (which contain the database of what the Installer has to do to uninstall old version files and install new version files) or it's just choking on one of the program installs and being all cryptic at us in the error message. We'll proceed on the assumption that the stuff you had uninstalled (and mysteriously showed up again) was put back during either one of the abortive QuickTime or iTunes installs.
So let's try going forward again. Probably best to print out a copy of these instructions, because there will be a few restarts along the way, and at least one stage where you shouldn't have your web browser running.
Uninstall any remaining Apple programs/componentry relating to iTunes again, as per the following document:
http://support.apple.com/kb/ht1925
(If you get any error messages on the uninstalls or file/folder deletions let us know what they say.)
Then empty your temp directory and restart, as per the first item in "General Installation troubleshooting" in the following document:
http://support.apple.com/kb/HT1926
After the restart, download and install the Windows Installer CleanUp utility:
http://support.microsoft.com/default.aspx?scid=kb;en-us;290301
Go "Start > All Programs > Windows Install Clean Up"
In the list of programs that appears in Clean Up, select any *Apple Application Support, Apple Mobile Device Support, Apple Software Update, BonJour, iTunes* and/or
QuickTime entries. Then click "Remove", and quit out of CleanUp. (under normal circumstances, this should remove any msi files still lurking about that might be mucking up our other attempts at installing.
Download and
save a fresh copy of the iTunes installer to your desktop. (Don't run the install on line, and don't start the install just yet.)
http://support.apple.com/downloads/
Restart the PC. Do not launch any programs (in case a browser or some other software has been grabbing hold of an iTunes or QuickTime plugin and has been interfering with the earlier installs). Disconnect from your network or the internet. Switch off any firewall, antivirus or antispyware applications (just in case they've been interfering with the earlier attempts at installs).
Now start the install by doubleclicking the iTunesSetup.exe you downloaded earlier. After the install attempt, you can reenable your security software and reconnect to your network/internet.
Does that install go through any better?