Slow Dell Laptop Now Won't Boot Or Repair

I'm working my sister's Dell Inspiron n5010 running Windows 7 Enterprise. It has been fine until this week when it became slow to the point of unusable. I had installed Microsoft Security Essentials on it for her, and she said one night when she shut it down, it installed updates. It finished and shut down. Next morning, she opened the lid and the screen was on saying "installing update 1 of 12, 0%" as if it had never finished and froze up from the night before. It finally finished and when it booted into Windows, I tried to run a virus scan, but it said Security Essentials was out of date and failed to start. I tried unsuccessfully to update it, and I couldn't get much of anything else to run either, it was so slow. I used my install disc to start fresh with a new install of Windows, and it seemed to fix it, until I got to installing drivers. before beginning on the drivers, I installed Security Essentials and MalwareBytes, ran scans, and they came up clean. As soon as I started installing drivers, though, it became slow, froze up, and I was back to square one. Now when it tries to boot to Windows, it gives two options: launch startup repair (which freezes) or start Windows normally (which also freezes). Booting from the install disc to start again with a Windows install won't even work now. As soon as it boots from the cd, it freezes.