1. You have a bad install disk. Try creating an ISO file of your dvd and either re-burn the disk, or create a Windows 7 install flash drive. Details on how to create the boot install flash drive can be found in this thread
2. You have a failing hard drive. Download and burn a copy of the Ultimate Boot CD from www.ultimatebootcd.com. Run some of the hard drive diagnostic tools and see if any problems are found.
3. You have a bad RAM stick. With the Ultimate Boot CD you downloaded earlier, run the MemTest86+ utility and again, see if any errors are detected.