Hi Anniemcb,
Welcome to the forum. I hope you'll find a lot of helpful information here.
I'm not sure I completely understand the problem. So please forgive me if there's a missunderstanding.
I'm assuming you're refering to downloading the Origin version of Sims 3? If so, I'd like to suggest you read this thread, written by Flynn Arrowstarr -
How-To: Backup and Restore The Sims 3 From Origin Downloader
http://www.carls-sims-3-guide.com/forum/index.php/topic,12079.0.html If you're using a physical disk all you need to do is take it and install it on the new computer. Using the same code you previously entered when you installed it on your old computer. No need to download anything other than updates.
Here's a helpful how-to on uninstalling and reinstalling Sims 3, written by Pam.
Start From the Beginning - Uninstall and Reinstall
http://www.carls-sims-3-guide.com/forum/index.php/topic,1103.0.htmlHope this helps,
MrsFlynn