For any number of reasons, a download of AutoCAD 2011 might be unsuccesful.
Security software, a long download resulting in corrupted files due to data loss, a power outage during the download, etc.
Autodesk offers a free trial version of AutoCAD 2011 on DVD, this is the full version just like the download. The disc has both the 32-bit and 64-bit versions loaded onto it.
As long as you have a valid serial number and product key, you can use those to activate the trial version.
Here's the url:
http://usa.autodesk.com/adsk/servlet/oc/offer/form?siteID=123112&id=12672865
We moved your post to a new thread in the Installation & Licensing forum where there is likely to be more active feedback on the topic.