Yes you can delete the def file from the core and run a download update to redownload the file. It does sound like a corrupt file.
I would also suggest you make sure you have the newest spyware patch installed.
This is a must if you are doing anything with spyware.
Can you copy the URL from the failed download and try it in the browser on the client machine?
What error does it show in the vulscan.log?
Thank-you both for replying. I deleted the definition in question and then dowloaded spyware updates; the file was recreated and I ran a scan successfully on a different machine. Tracy, sorry I followed Aaron's advice before I saw your post. I actually had to give the laptops I was having a problem with back to their owners. So I can't get the logs right now.
Thanks again for the help