Write error on file "_themes/customtheme"

So you created and deployed a new theme following these steps (http://msdn.microsoft.com/en-us/library/aa979310.aspx) and when you apply your theme you get the following error message " Write error on file "_themes/customtheme". You've doublechecked the spelling for everything, you don't have any invalid characters in the title of your theme (even an underscore or a number in the title can cause problems). You've even tried changing the codepage from 65001 to 65002 and that doesn't work.
Here's what to do:
Search the SharePoint logs (12\logs\) for "thumbs.db"; you'll probably see a line in there that says something like "invalid URL _themes/customtheme/thumbs.db". So what is thumbs.db? it's a hidden system file that is used to cache thumbnails for image files, such as the image files that you copied into your theme folder.
Next, in windows explorer click Tools > Folder Options > View. About 8 lines down you'll see a checkbox for "Do not cache thumbnails". Click it.

Then delete thumbs.db or recreate the folder.

Finally, perform an IIS Reset and your beautiful new theme will (hopefully) work.