If you are having trouble re-imaging a MARS appliance there are a few things you can do to try and fix the issues.
First, make sure you have enough disk space free on your computer. IE downloads to a temp directory then copies it to the directory you want to use. Be sure you have 2x the space you need.
Next, try disabling your anti-virus software temporarily. It may be choking on the large file.
Trouble using the image:
The first thing to do is to check the download was successful. Use a program to calculate the MD5 hash of the .iso image and compare it to the hash posted on Cisco.com. HashCalc is a good program for this, and it's free.
Next try slowing down the DVD burn to 2.4 or 1.0x.
After that, try different media or a different program.
Burnatonce is a good alternative program if you are running Windows XP - http://burnatonce.net - and want to burn a bootable DVD.
If not successful there then try a different computer to burn the DVD.