Error Message:
 
Retrieving the COM class factory for component with CLSID {00024500-0000-0000-C000-000000000046} failed due to the following error: 80070005 Access is denied. (Exception from HRESULT: 0×80070005 (E_ACCESSDENIED)).
Solution:
 
This is a COM permissions problem.
 
Start > Run > dcomcnfg (or “mmc -32″ and then add the Component Services snap-in manually if you can’t find the app under step 3)
Navigate to Component Services > Computers > My Computer > DCOM Config
Locate the MS application giving you trouble (e.g.: “Microsoft Excel Application” for Excel or “Microsoft Word 97 – 2003 Document” for Word)
Right click > Properties
On the security tab: Select Customize under Launch and Activation Permissions and click Edit…
Add the account under which the site is running (e.g.: Network Service) and assign Local Launch & Local Activation permissions
 
Error Message:
 
[COMException (0x800a03ec): Microsoft Excel cannot access the file ''. There are several possible reasons:
• The file name or path does not exist.
• The file is being used by another program.
• The workbook you are trying to save has the same name as a currently open workbook.
Solution:
 
This seems to be an issue with Excel/Word/etc not having access to a profile when being started via the interop route.
 
Open Windows Explorer
Depending on whether you installed a 32bit or 64bit version of office you will need to do one (or both) of the following:
32bit Office installation: Navigate to C:\Windows\System32\config\systemprofile
64bit Office installation: Navigate to C:\Windows\SysWOW64\config\systemprofile
Verify the folder "Desktop" exists (create it if it's not there)
Right click > Properties
On the security tab: Add the account under which the site is running (e.g.: Network Service) with default permissions (Read & execute; List folder contents; Read).