S.M.A.R.T. Error remains after swapping drive & motherboard

Hello everyone,

I'll try to make this short.

My Desktop PC (running Win7) had some issues that grew in number over time. This would include windows of folders freezing (the rest still running) and games lagging that used to run fine.
At some point, I received a message whenever booting: S.M.A.R.T. error, HDD status BAD, backup and replace. From that point on, the Computer would sometimes freeze or automatically restart when trying to boot, sometimes became often, often became always.

I get to the sequence of Windows loading, but never past that. The SMART error shows up every time.

I tried:

-Swapping the hard drive and SATA cable with a brand new one (both "Western Digital") and installing a fresh Windows 7 (nothing else)
-Swapping the motherboard with a new one (both "ASUS").
-Trying the 4 RAM bars individually.

After all that, the error still remains.

The parts left that I think could be responsible are:

CPU (AMD Phenom II X6 1100T)
Power adapter
[CD drive + card reader]

Any input is appreciated. I'm desperate, having spend over 100€ on not fixing the problem.

Thanks in advance, Bagatoli