Thumbnails are used to show a preview of image without opening the image in any Image Viewer or Editor. To Show thumbnails of pictures in Windows XP you have to enable thumbnail View. Once you enable Thumbnail view windows create a Thumbnail Database file (thumbs.db) in same folder and cache the thumbnails of images into this file. This way windows will show you preview of images faster next time you browse through same set of pictures.
If you like the Thumbnail view of images but not satisfied with quality or size of the thumbnails you can change default setting of these by using following registry hack. (Always backup your registry before making any changes).
However Before changing these settings be sure that your computer have enough RAM and processing resources to handle these. As Thumbnail views can use a lot of system resources making your computer slow. If you increase the quality and size of thumbnail view, Browsing through files will require more time as building thumbnail is quite a slow process. Also if you have very large folders with thousands of images in it. Then it will be very time consuming and also your thumbnail cache can grow big.
Anyway Here the registry setting which needs to be setup in order to change the size and quality of thumbnails. (this works on XP and Vista Already have 3-4 different thumbnail views in built).
Open the registry and navigate to :
HKEY_CURRENT_USERSoftwareMicrosoft WindowsCurrentVersionExplorer
Create a new DWORD value called ThumbnailSize, and set the value between 32 and 256, and/or create another DWORD value called ThumbnailQuality, and set the value between 50 and 100.
First DWORD value defines size, higher the value, bigger the size of thumbnail. Also second keyword defines quality higher the value better would be quality, but of course it can not be better than your actual image 
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The key details are given below once again –
Key Details :
USER Key: [HKEY_CURRENT_USER Software Microsoft Windows CurrentVersion Explorer]
Value Name: ThumbnailSize
Data Type: REG_DWORD (DWORD Value)
Data Value: 32 – 256USER Key: [HKEY_CURRENT_USER Software Microsoft Windows CurrentVersion Explorer]
Value Name: ThumbnailQuality
Data Type: REG_DWORD (DWORD Value)
Data Value: 50 – 100
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Hi,
There is no thumbnail size or quality in the directory specified on my machine. I searched the registry which could not find either word. Really perplexing as I have done this before on my other system with XP
Cheers
Anthony
Interesting.
Sounds like what I was thinking when I was trying to get a disk formatted on an older XP machine.
Thanks for posting.
If it doesn’t exist.. that is most likely why Author said “Create a new DWORD value called ThumbnailSize, and set the value between 32 and 256, and/or create another DWORD value called ThumbnailQuality, and set the value between 50 and 100.” Your eyes need their CCDs checked, bad scan lol, or you moved your mouse wheel down the instructions a little too fast