After you download the iso (I downloaded smeserver-7.0.iso) make sure you get a MD5 checker and compare it to the MD5 provided on the server
Then use GOOD CD Media and Burn it using your slowest speed ( I used 4x)
Most of the people I know buy the cheapest hardware and build a Server for their client, and they usually skimp on the CD-Rom or DVD rom.
Make sure your Server has a good CD or DVD Rom. Somestimes a DVD drive reads the CD too fast causing errors.
After just making sure you create the CD right i was able to install SME-Server on a 160GB SATA Drive as easy as ever.
Also, don't give up if it still dosn't read the CD, I've read that sometimes people have to try 5 or 6 times before it installs 100%