I have an alternative out of the box idea. I ran into this same problem with a totally unreadable Windows 7 COA. I noticed upon extemely close examination that the COA actually leaves very small indentations. If you take a very small amount of acrylic paint on your finger and very very lightly rub the area the numbers will start to reappear. It took about 30 - 45 minutes of tinkering and examining but now I can read all of my numbers. Only one number is within question. For clarity I could only read about 3 or 4 numbers / letters before this process. The numbers you see below were completely unreadable as was the words "Product Key". Hope this helps someone out. By the way my machine is totally dead right now without a hard drive so no software options will help me. I am sure there are others just like this. [IMAGE]
If your Windows Product Key label is worn off or unreadable you can forget about getting any help from Dell or Microsoft. You are basically on your own. Microsoft will send you to Dell. Dell will send you to Microsoft. Microsoft will try to charge you money. Dell will try to charge you money. You will spend an hour chasing your tail and you will get nowhere. It's a very simple and very common problem. It is bewildering to me that there is not some sort of procedure in place to deal with a worn off or unreadable COA label.  
Cannot read Windows product key on sticker?