Well I just went to apple.com and tried downloading both versions, iTunes.exe IS the windows version the Mac version is iTunes.dmg so it's not THAT problem...
Have you run scandisk lately or defrag? maybe there's little space left on your drive so it keeps getting downloaded onto a section of the drive having problems...?
What Operating system are you using? all updated/patched? what type of computer? and have you tried Apple - Support
or Apple - Support - Discussions - Forum Home
the discussions board, there's a section on the boards for iTunes for windows and in their doing a search for "valid win32" came up with many many many threads and the one solution that appears to work is
"was also able to solve Win 32 error by deleting my temporary internet files. Just go to Internet Explorer, select Tools, then Internet Options from the menu bar. Click on the button called Delete Files. Then do the download process again for the iTunes upgrade. Hope it works for you too."