Go back and check the MD5 hash of the .iso you downloaded. I was running into some issues with BT5 where I was too anxious to play around and forgot to check the hash of the .iso. Booting and running BT5 in live mode worked fine, but when I tried to install to disk, it failed. After going back and checking the hash on the file I downloaded, I found it was different, so I had to redownload the .iso. After getting a good download, I was able to install to disk without problems.