How to change the disk space left warning in Windows 7?

There is literally the same question on Microsoft Answers (I guess you asked it there too) and the answer of Swathi B (Microsoft Support Moderator):

This is by architecture; unfortunately there is no option to change the percentage for the low space warnings on the computer.

You may provide the feedback about adding any new feature to the Operating System on the link: http://connect.microsoft.com

You can't change the percentage because it's hardcoded. Well, just get rid of those bars and voila - problem solved.

Source

1. Run regedit.exe as administrator
2. Open HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Drive

3. Change the values for both TileInfo and PreviewDetails keys by removing the phrase "System.PercentFull;" (without the quotes)

   TileInfo key before and after

   prop:*System.PercentFull;System.Computer.DecoratedFreeSpace;System.Volume.FileSystem
   prop:*System.Computer.DecoratedFreeSpace;System.Volume.FileSystem
   

   PreviewDetails key before and after

   prop:*System.PercentFull;System.FreeSpace;System.Capacity;System.Volume.FileSystem;*System.Volume.BitLockerProtection
   prop:*System.FreeSpace;System.Capacity;System.Volume.FileSystem;*System.Volume.BitLockerProtection
   

4. Open My Computer. You should be able to see the absence of the color bars. If you still see them, just restart Windows Explorer or Windows itself.

Found your exact question on the microsoft answers site, and it's not possible: http://social.answers.microsoft.com/Forums/en-US/w7performance/thread/b42d61fd-9f5b-41d3-939a-83724aabab71

Would be nice to see it in a newer version of Windows.