Today, April 19, 2011
Okay ... not likely to be a Windows Update issue that has just recently come on, then.
The 80070002 errors acn be associated with damaged files or missing permissions, and conceivably software interference too. So let's throw a bunch of basic principles troubleshooting procedures along those lines at the problem.
First, I'd try running a disk check (chkdsk) over your C drive, or whichever drive you install your program files on.
There's Vista instructions in the following document.
http://windows.microsoft.com/en-us/windows-vista/Check-your-hard-disk-fo r-errors
Select both Automatically fix file system errors and Scan for and attempt recovery of bad sectors. You'll probably need to schedule the scan to run on restart. The disk check should take quite a while to run (if you're running it on the C). If it quits after a few minutes or seconds, something's interfering with the scan.
After that completes, download a fresh copy of iTunesSetup.exe from the Apple Website and save it to your hard drive. (Don't run the install on line, and don't start the install just yet.)
Restart the PC. After the PC restarts, do not open any applications. Disconnect from the internet and/or your network. Now disable all your security software (firewall, antivirus, antispyware).
Now start the iTunes install by right-clicking the iTunesSetup.exe and selecting "Run as administrator".
(Reenable all security software prior to reconnecting to the internet and/or your network.)
Any better luck with the install that time?