KB 944105: How to customize application pages in the Layouts folder in WSS 3.0 and MOSS 2007
This recently published and just updated (today) KB article addresses a very frequently asked question about how to customize SharePoint's "Layouts pages" in a supported manner.
More Information
Modifying the files that are installed by SharePoint is not supported. However, there are some scenarios in which you may have to modify these files to achieve consistent branding or other customizations. When you modify these files, you must consider that they may be replaced by future updates and service packs. Additionally, there may be complications when you upgrade to later versions of the product. Keep backup copies of all customized files in case they are overwritten by an update. Microsoft product support will provide commercially reasonable support for help with modifications but will be unable to provide product changes or hotfixes that result from modifying the files that are installed by SharePoint.
KB Article
Go to the KB article at http://support.microsoft.com/kb/944105/en-us for the details.
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