What are Thumbs.db files for? How can I prevent them from being created?

thumbs.db is where the thumbnail images for files are stored. They will only be visible if you have your folder options set to display hidden and system files. They are not viral.

If you delete a thumbs.db file, the next time you open that folder, Windows will have to re-determine the thumbnail for each file.

Also, in Folder Options, selecting "Do not cache thumbnails" will prevent the thumbs.db files from being created.

However, these files help the computer navigate through your directories more quickly, so unless you have a specific reason for not needing or wanting them, just leave them alone.

From https://secure.wikimedia.org/wikipedia/en/wiki/Windows_thumbnail_cache

On Microsoft Windows operating systems (starting with Windows 2000), a thumbnail cache is used to store thumbnail images for Windows Explorer's thumbnail view. This speeds up the display of images as the smaller images do not need to be recalculated every time the user views the folder.

...

Thumbs.db files are stored in each directory that contains thumbnails on Windows systems.

As an addendum to the other answers, to prevent creation of thumbs.db in Windows Vista and above (7 etc) you should not go into folder options and disable thumbnails, you can have your cake and eat it too.

Just open the Local Group Policy Editor (+R and enter gpedit.msc) and go to;

User Configuration -> Administrative Templates -> Windows Components > Windows Explorer

_{or File Explorer for win 8}

And edit Turn off the caching of thumbnails in hidden thumbs.db files. Enable it and hit OK.

To delete ones already created you can right click -> Properties on the drive from My Computer, hit Disk Cleanup, tick only Thumbnails and hit OK.

If you cant access the LGPE (e.g. MMC could not create the snap-in) or it just plain didn't work you can dabble in the registry;

HKEY_CURRENT_USER -> Software -> Microsoft -> Windows -> CurrentVersion -> Explorer -> Advanced

set DisableThumbnailCache to 1

If DisableThumbnailCache doesn't exist, create it as a new DWORD

_{reboot may or may not be required}