Downloading Issue in Serif Web Plus X4

I have a Windows 7 laptop with 4GB RAM, 1TB Hard Disk Drive and i7 Processor. I also have an Acer Aspire T761 desktop PC with Windows XP SP3 running on it, 2GB RAM, 160GB Hard Disk Drive and Intel Pentium D Processor.
I am trying to download Serif Web Plus X4. After downloading, I go to My Computer and double-click the application to start the installation. But a message appears stating that, I need to select a program to launch the files.
But I am getting confused since there are lots of applications to choose from. I downloaded Serif Web Plus X4 on my laptop and successfully installed it without having errors. However, I cannot install it on my desktop PC.
Can someone please help me?
