Hi Alberto,
I did some homework and yes the problem is a service trying to talk to a DDE server.
So a solution may not be possible but you could try:
#1 on the same "Log On" tab, change it to use a user account (the account you are logged in via) and not the system account, make sure you also tick interact with desktop. Then start the program that is going to be accessed via DDE, then restart your service. See if it works.
#2 starting excel (well its DDE server via the API call CreateProcess() or CreateProcessAsUser(). This may involve hacking the template, and i suspect much pain.
Maybe you might want to consider going back to .CSV for your spreadsheet (if its just data and not formulas).
When you think about how DDE works its really not suitable in a server/service environment.
Regards
Bill