Sitecore access to the viewstate folder denied

If you are unable to load the Sitecore desktop and get errors pertaining to access being denied on the Data\viewstate folder (and its many child folders), it's a permissions issue...

To allow file uploading and modification of the site, grant the ASP.NET user Read/Write rights to the following folders:

• /data
• /upload
• /temp (if this directory does not exists immediatly after installation, create it).
• /sitecore/shell/Applications/debug (if this directory does not exist immediately after installation, create it).
• /sitecore/shell/Controls/debug (if this directory does not exists immediately after installation, create it). 
• /layouts (If Developer Center will be used to create and modify layouts and sublayouts).
• /xsl (If Developer Center will be used to create and modify XSL renderings).
• /App_Data/

In order to maintain the search indexes ASP.NET and IUSR_* users also require the modify access rights to the following folder:

• /indexes

Both the ASP.NET and IUSR_* users require Read and Write access rights to the following folders:

• /audit
• /logs
• /viewstate
• /mediacache
• /diagnostics

Where IUSR_* refers to IUSR_MachineName.