Have had to do this twice now with all of the problems as above.
First back up any Itunes media (music, videos etc) and Iphone backups you may want. (this is just insurance)
Only way to delete a troublesome Itunes from PC without reinstalling windows is to delete ALL Apple software using control panel as Chris said. But he left out Apple software update, Apple application support, Apple Mobile Device support, Bonjour, MobileMe Control Panel, Apple Airport Utillity.
Dont know about utils like Rinse and Tuneup as I only recently started using them. Same for Airport as I only recently got my Airport Extreme (best modem router I have ever had).
Follow this with Revo Uninstaller (free and highly recommended) to get the leftover bits from the registry.
A bit scary I know from experience.
Restart PC.
Check task Manager for any Apple Services and or Processes. (Stopping them here does not help)
If any running you have missed uninstalling something. Go to Control Panel again and delete the one you missed.
Restart PC and check again before continuing.
Check again that all is uninstalled then download Itunes to PC and reinstall.
Pray.
Reinstall all of the others you need.
This, although painful, should allow you to keep all your windows setup.
Of course reinstalling Windows will work as well if you dont have years of customisation to keep
Most people who say to reinstall windows have no idea what that might mean to a mission critical business PC.