If your Laptop is Windows 7 Pro, Enterprise or Ultimate, you can install 'XP Mode' which is a combination of Virtual PC and a full copy of XP direct from Microsoft for free from here
If you don't have one of the three versions mentioned above, but instead have the Home Edition, you can install VirtualPC from here, or VirtualBox from here, but you will need a spare copy of XP to install
I use VirtualBox as it has more features, but is slightly more complicated to setup
Because the installation of Diagbox can be a real pain and if it goes wrong, you end up having to start again with a fresh install of XP, VirtualBox allows you to make copies of the initial install of XP, so if anything does go wrong, you can start again with the Diagbox installation on another instance of XP
This saves the time in having to install XP again
If you don't have one of the three versions mentioned above, but instead have the Home Edition, you can install VirtualPC from here, or VirtualBox from here, but you will need a spare copy of XP to install
I use VirtualBox as it has more features, but is slightly more complicated to setup
Because the installation of Diagbox can be a real pain and if it goes wrong, you end up having to start again with a fresh install of XP, VirtualBox allows you to make copies of the initial install of XP, so if anything does go wrong, you can start again with the Diagbox installation on another instance of XP
This saves the time in having to install XP again