Damaged metadata

Hello

I've encountered a problem while processing some images with exiftool.
While my Windows explorer (Win7) didn't show any signs of corrupted data, with exiftool I've discovered that the XPcomment tag was corrupted/damaged (it was the tag which I was working with).



As you can see on the left the XP Comment tag appear with weird symbols while on the right the explorer window shows the data correctly.

What could cause this problem and how I can fix it?
Thanks.

Yes actually I had those images restored from an online backup in Picasa and I have to use special characters since in Italian almost every day of the week have a " ì "

Monday -> Lunedì
Tuesday -> Martedì
Wednesday -> Mercoledì

And so on...

I can easily write those special characters with exiftool just by setting the HTML language with the -E command.

The "i" or "ì" itself its not a problem since a lot of images I have also have it and they're not broken.