What causes computer freezing?

[ScottyP431]

I have a two year old laptop. After about year 1 it began freezing occasionally where all functions stop and the screen is locked as an image of the last second where it was functioning. This has started happening a lot more recently. The computer has had its hard drive completely wiped clean and had the OS (XP pro) reinstalled twice over the last year for other problems but neither time did the freezing stop. So, the questions
1, Am i right that because completely reformating the hard drive didn't fix it that this has to be a hard ware problem?
2, What could cause such a problem?
3, has anyone had any success getting their manufacturer to replace product or fix this problem under warranty?