You need to determine whether it is your hard drive or software corruption.
If you have a spare hard drive laying around, unplug the current one and attempt to install ANY operating system (even Linux) just to see if it will happen with no issues.
That proves your computer works.
You can also test your current hard drive by installing it in another computer (can use a USB case) and performing CHECKDISK (scan folders AND rebuild bad sector table).
You may get away with simply reinstalling your Windows 7 OEM disc on your current hard drive.
*When you reinstall Windows 7:
1. Run checkdisk (check file system AND build bad sector table)
2. Run diagnostics from the hard drive company
3. ACTIVATE your newly installed copy and immediately->
4. Create a backup IMAGE (can use WD version of Acronis if you have a WD hard drive)
**You can also download a bootable diagnostic disc to see if your current hard drive is okay.
There are some tips online on how to repair Windows startup but it's probably best to reinstall.
There's no need to buy a different copy of Windows.