I feel your pain! and BTW I think Apple really messed this update up - any way....
I'm running Windows XP no problems to date with iTunes, iPhone, iPad, etc.
The good news is that I cobbled together an approach that worked taking suggestions from this and other Apple support groups.
To be clear my problem started when I opened iTunes 10.4 and it asked me to update to iTunes 10.4.1 and I started the update - I’ll never do that again without a lot of thought.
The update failed.
I read the support group info and tried first the recommendation of a specific delete sequence. Deleted iTunes - no problem. On trying to delete QuickTime I got the message (paraphrasing) ...the application is not accessible, etc, tried to browse to the folder with QuickTime.msi., found it, but no luck.
I want to make clear that I am not a sophisticated computer user, but rather a puzzle solver and sometimes I make horrible mistakes so no one should follow my steps with the expectations that the same result will be accomplished.
Steps??? I wouldn't call it that but more or less what seemed to produce the results I hoped for.
I downloaded:
http://download.cnet.com/Revo-Uninstaller-Pro/3000-2096_4-10687648.html
I found this program worked under circumstatnces I'll describe later.
next:
http://www.utililab.com/registrycleaner/
cost me $14 and it may have carried a malevolent search eng. - ASK. Check what you are accepting when you pay if you go this route.
Last and what seems like the best advice - reboot in "safe mode". If you don't know how to do this it is probably better to take your machine to someone who does.
In safe mode I oppened the uninstaller program and took ASK out and tried Q Time - no luck on the later.
Rebooted
Did regestry cleaner - actually every time I did something I did the registry cleaner.
In safe mode opened:
1 - All Programs
2 - System Tools
3 - Disk Clean UP
You will get a dialog box - which disk? enter the hard drive that held iTunes and click
The next dialog box opens with a lot of choices - I checked everyting but most importanty (according to various sites) temp files.
At that point and close to midnight I figured one more try would not hurt so, I downloaded a new copy of iTunes (deleted every other copy I had previously downloaded including stand alone QTime) and --- it worked.
I am up and running, iTunes works - ASK is gone and so far my library works, iPhone and IPad synchro with iTunes works --- so...
Good luck