My computer keeps freezing & ctrl alt dlt doesnt work, help ?

Its a Toshiba laptop, about a year old. & i have windows vista. I have taken excellent care of it and it has been absolutely flawless up until a few days ago. It still has 100 GB of space available, because like I said, i am very diligent in getting rid of junk and not downloading unnecessary things. I have run disk cleanup, defragged it, & ran a virus scan. I have also used CCleaner, and CCleaners registry scan. All of these tests are showing no problems, yet my computer keeps freezing. It is not slow at all, and it does not freeze on any specific website. At random times, itll freeze and then CTR ALT DLT wont even work. what could this be?