You will have saved the downloaded file in a folder on your hard drive.
You'll need to search to locate it, but once you have you can transfer it to a memory stick or to a CD.
Then move the file into the 'Downloads' folder on your Vista machine and double click on it. The installer will run, and away you go. You'll be prompted for your installation key, and here's hoping you kept a note of it, or kept a copy of the email you received at the time with the key in it.
Remember that your license only allows you to run the software on one machine.