1. At the start menu, type cmd.exe. You will see "Command Prompt" in the results list. Right-click on it. A context menu-bar will show up at the bottom of the screen. Click on "Run as administrator". Click "yes" to the User Account Control prompt. Command shell will start.
2. Let's see your LOCALAPPDATA setting. Type...
C:\>set LOCALAPPDATA
LOCALAPPDATA=C:\Users\Paul\AppData\Local
3. Let's see the contents of the WinStore_cw5n1h2txyewy folder. Type...
C:\>dir "%LOCALAPPDATA%\Packages\WinStore_cw5n1h2txyewy"
Volume in drive C is System
Volume Serial Number is CE28-C1D0
Directory of C:\Users\Paul\AppData\Local\Packages\WinStore_cw5n1h2txyewy
03/18/2013 11:24 PM <DIR> .
03/18/2013 11:24 PM <DIR> ..
03/18/2013 11:25 PM <DIR> AC
03/18/2013 11:24 PM <DIR> Settings
4. Do you have the AC folder? If not type...
C:\>md "%LOCALAPPDATA%\Packages\WinStore_cw5n1h2txyewy\AC\PRICache"
5. Now check the ACEs (Access Control Entries) in the ACL (Access Control List) for the AC the folder. Type...
C:\>icacls "%LOCALAPPDATA%\Packages\WinStore_cw5n1h2txyewy\AC"
C:\Users\Paul\AppData\Local\Packages\WinStore_cw5n1h2txyewy\AC S-1-15-2-2608634532-1453884237-1118350049-1925931850-670756941-1603938316-3764965493:(OI)(CI)(F)
NT AUTHORITY\SYSTEM:(I)(OI)(CI)(F)
BUILTIN\Administrators:(I)(OI)(CI)(F)
PAULBROHOME\Paul:(I)(OI)(CI)(F)
Mandatory Label\Low Mandatory Level:(OI)(CI)(NW)
Successfully processed 1 files; Failed processing 0 files
6. Do you see the S-1-15-2.... and "Mandatory Label\Low Mandatory Level" ACEs in the list? If not, type...
C:\>icacls "%LOCALAPPDATA%\Packages\WinStore_cw5n1h2txyewy\AC" /grant *S-1-15-2-2608634532-1453884237-1118350049-1925931850-670756941-1603938316-3764965493:(OI)(CI)(F) /setintegritylevel (OI)(CI)L
processed file: C:\Users\Paul\AppData\Local\Packages\WinStore_cw5n1h2txyewy\AC\
Successfully processed 1 files; Failed processing 0 files
6. Try to launch the Store app