Startup Repair + Windows Installation

Do try to attempt to clean install first before resorting too buying new hardware. May need to change your boot order to DVD drive - first boot device.

As far as, replacing the hard drive. Yes - you can reinstall OEM on a replacement hard drive in the same computer.

When it's an OEM license
1. You can reinstall as many times as needed on the same computer. 
2. You can replace the hard drive and reinstall Windows OEM on the same computer.
3. You can not move OEM to a different computer.

Difference Between a Full/Upgrade/Retail License and an OEM/System Builder License.
OEM licenses of Windows:
1) Are bound to the first computer installed on and can't be moved to a different computer.
2) Can not do an in place upgrade from older versions of Windows, only a clean/fresh install is possible.
3) Offer no free support from Microsoft.

Hope this helps'
John

I already tried to reinstall windows 7. I tried to download ISO file of Windows 7 from Microsoft's page. I entered the product key and got this:

Error

The product key you entered appears to be for software pre-installed by the device manufacturer. Please contact the device manufacturer for software recovery options.

I got the product key off a sticker on my laptop. I bought laptop new with preinstalled Windows 7. So the biggest problem right now is that I can't install Windows.

Well, there's always the option to look at one of those bias reviews when making a chose.
Best SATA Drives of 2016 - Budget SSDs With High Performance & Value

As Bigmck pointed out, it's all about your budget.

John

Thank you for all the replies. In the end I bought Samsung 850 evo SSD, which was a little over my budget but I think I'll manage

Please, let us know how everything turns out. :)

John

Yesterday I took the laptop in for service and they'll install windows. I hope I won't have any more problems after I get it back :)

Glad you were able to get it sorted.

We can consider this issue closed.