I'm getting an error message on my desktop app

Dropbox has been working great on my Win 7 32 bit laptop for at least 6 months.  Recently, I've been getting an error trying to start Dropbox stating 'This is usually because of a permissions error.'  I tried the steps at https://www.dropbox.com/help/72 (unlinking the laptop, manually running icacls) but the same error comes up when I try to start Dropbox.

I searched the web for solutions, but it seems that most people having this issue are Mac users.  The same is true for people in the Dropbox forums.

So I used Control Panel to uninstall Dropbox, then File Explorer to delete every Dropbox directory, then Regedit to delete every trace of Dropbox from the registry.  Downloaded the Offline installer and ran that.  It appears to install correctly, but the last step is starting Dropbox, and it throws the same error.

Here is the log it generates:

[get_extra_trace_info failed]
Traceback (most recent call last):
  File "dropbox\client\main.pyo", line 614, in boot_error
  File "dropbox\client\main.pyo", line 3759, in get_dropbox_path
AttributeError: 'NoneType' object has no attribute 'config_dict'

Original traceback:

Traceback (most recent call last):
  File "dropbox\client\main.pyo", line 4065, in main_startup
  File "dropbox\client\main.pyo", line 1788, in run
  File "dropbox\client\main.pyo", line 863, in startup_low
  File "dropbox\client\multiaccount\instance_database.pyo", line 848, in __init__
  File "dropbox\client\mapreduce.pyo", line 64, in __init__
  File "dropbox\client\mapreduce.pyo", line 110, in new_user_key
  File "dropbox\keystore\keystore_win32.pyo", line 65, in store_versioned_key
  File "dropbox\keystore\keystore_win32.pyo", line 72, in _store_versioned_key
  File "dropbox\keystore\keystore_win32.pyo", line 152, in _encrypt_payload
  File "pynt\helpers\crypt.pyo", line 27, in protect_data
FormattedWindowsError: [Error 5] Access is denied.

So I tried icacls again, and they all run except this: 

icacls "%PROGRAMFILES%\Dropbox" /grant "%USERNAME%":F /T

which throws 'C:\Program Files\Dropbox: Access is denied.'  Which isn't a surprise, given Windows 7's restrictions on Program Files.  I had to add my user to \Dropbox and \Dropbox\CrashReports (Full control) but Dropbox continues to throw the 'permissions' error when it tries to start.  My profile has Full control over \Windows\Temp.  I'm out of ideas.