Excel needs the Desktop

To make Excel automation works from IIS:
Excel uses the "Desktop" folder for something.
Even though IIS execution right personalized to an user specified in the application pool advanced settings, Excel still executes in the system environment, so, we need to create the desktop folder for it:

C:\Windows\System32\config\systemprofile\Desktop
read more at:

http://social.msdn.microsoft.com/Forums/en-US/innovateonoffice/thread/b81a3c4e-62db-488b-af06-44421818ef91?prof=required

Also, give proper rights to the folder "C:\Windows\System32\config\systemprofile\" and to the Excel's DCOM properties (go "Administrative Tools/Component Services") to the user which executes Excel

To run the DCOM config:
When the office has the same bits as the OS:
DCOMCNFG.exe

When 32bit office on 64bit OS:

mmc comexp.msc /32

Note! Even if the caller 32bit and the Excel is 64 bit, it should work fine, since the Excel object is called via DCOM.

Also! The Excel need a valid accessible printer to produce documents.
Make sure the default printer has enough permissions which allow the IIS user to print on it.