Windows XP stores the size and quality it uses to display thumbnail images at the HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer key.

The ThumbnailSize Value Name, a REG_DWORD data type, determines the size of the thumbnail image. The valid values range from 0x20 (32) - 0xFF (255). The default data value 0x64 (100)

The ThumbnailQuality Value Name, a REG_DWORD data type, controls the image quality. The valid values range from 0x32 (50) - 0x64 (100). The default data value is 0x5A (90).

NOTE: You must log off and log on for these setting to take effect.