There are many versions of Vista and two versions of the Service Pack - SP1 and SP2. Before you attempt to upgrade to a latest version of Windows Vista, first determine what version you have. Follow these steps:
1. Click on the Start Button.
2. Right-Click on the Computer.
3. Click on Properties.
Properties will show you a lot of useful information. For upgrade purposes what you want to know is the Service Pack version and whether your operating system is 32 bit or 64 bit.
In the provided example, the computer has Vista Home Premium, Service Pack 1; also, it is the 32-bit version. If this was your computer you would download and install the 32-Bit Version of Vista Service Pack 2 (SP2) from Microsoft's web site.
For more information, read the article "Upgrade Your Computer to Windows Vista SP2".