Windows 7: no icons in Explorer

I really don't know if this happened recently or if it's always been this way, but I just noticed that the little program-icon that normally (in Vista and XP and....) appears associated with a file, is empty.
To make sure you understand, if I've saved an html file, the little icon is a firefox icon because that's my default browser.
And, if it's an excel file, it's the excel icon.
I've gone into the TOOLS>FOLDER OPTIONS>VIEW tab
and the Display File Icon on Thumbnail box is checked.
I've rebooted.
When I look at the property of any html file, for example, the PROPERTIES window's OPENS WITH shows a Firefox icon.
Help!

Hello Newmarket,

You might see if rebuilding the icon cache may help with this.

Hope this helps,
Shawn