If you are looking to install or up-grade the your computer to Windows Vista then the first thing you need to look at is the Hardware compatibility, I understand that you are using Acer Aspire T180 computer which must have had very high configuration, however I recommend you to please check if the system is 100% compatible with the Windows Vista software as there is no harm in just making sure of what it is rather than running into issues later so with the help of the link below you can download and run the Windows Vista Upgrade Advisor to know the system compatibility.
Then once you know that the system is compatible with the Vista, you can now start the installation wizard. However do not worry, about the installation wizard asking you where to get installed as it is quite normal, you will have to provide the system with the hard-drive information, for it to get installed. The best way is to go with the default option that you see on your screen. Here are few links below through which you can download the guide for how to Up-grade to windows vista as well as for How to install Vista step-by-step
. Download the guides and use them for you assistance.
Windows Vista Up-grade/ Migration Guide
Windows Automated Installation Kit