All versions of Windows store crucial system and program information in the registry. On some versions (Windows 2000 and NT), the size of the registry is restricted by a setting. If the registry size reaches this maximum set value, an error will be displayed. If you are running Windows 2000 or NT, you can manually increase the maximum registry size. See this article  for details.
Windows XP and Vista have removed this limitation. There is no maximum registry size setting. If you are running Windows XP or Vista, and get a error message about the registry exceeding its allowed size, you will need to take other measures:
The primary culprit is usually Nvidia drivers. If you have a Nvidia graphics card, or a motherboard with a Nvidia chip-set, you have Nvidia drivers installed. You should go to the Nvidia website  and download the latest drivers for your graphics card or motherboard. Then, use “Add/Remove Programs” to uninstall your current drivers. After uninstalling, reboot your computer and proceed to install your newly downloaded drivers.
If you still receive the error after reinstalling the Nvidia drivers, or if your system doesn’t have any Nvidia drivers installed to begin with, download and run NTREGOPT . This program will optimize your registry. While the program has never caused me any trouble, you should make a backup of your computer before running just to be safe.
Finally, if all else fails, you may need to perform a repair install of Windows.