This is strange indeed and sounds like you have not been able to fix this issue from when you had problems with double output with the cgi scripts in your first post on these forums last year.
This may be a browser issue - do you have another browser installed that you can test with, if not then you should perhaps install one.
Lets have a clean start so we can perhaps troubleshoot this issue.
Because you have installed and then reinstalled again I suggest you do this again only this time follow these instructions or you may have the same problem occur again.
Open the XAMPP Control Panel (XCP) and stop all components and remove any ticks in the Svc boxes.
If the tick can't be removed then no problem as the next instructions will take care of that.
Close the XCP.
Now go to start>Run...
typesc delete mysql
then typesc delete apache2.2
Next delete the xampp folder completely.
Now download the EXE (7zip) self extracting version of XAMPP.
Extract the files to the C:\ drive and they will be placed in C:\xampp folder in the correct place.
Now run xampp\setup_xampp.bat file to set all the paths correctly.
Next run xampp\xampp-control.exe and start Apache and MySQL.
Go to http://localhost/
and select your language.
At the Welcome page test all the demos work correctly and you don't get any errors or double output.
When you are satisfied that XAMPP is working as it should then try and install your Joomla again afresh.
Let me know back how it all goes.