If you have any kind of warranty from that shop I'd keep taking it back til they fix or exchange.
It may be a internal hardware problem that is getting ready to break then you'll be stuck with another bad machine..

If it has a new hard drive then it should have a clean install of the Win 7- unless they cloned the bad drive for the install or used a bad install disk..

I's ask if it was a clean, fresh install of the Win7.
If so ask what hardware could be causing the lock/shut downs.

Something is not right - it shouldn't be doing those things

I think it's a Win7 bad installation - or funky internals going bad..motherboard, cpu ram, etc..