"Imminent HDD failure" message from Windows?

Are you sure it's from Windows? This looks a lot like the error message usually put out by the BIOS during booting when it gets a SMART warning from the HDD. If your BIOS is set to Enable SMART warning monitoring, that may be where the message comes from.

Modern hard drives have a semi-hidden set of built-in functions to handle drive problems and prevent data loss. Within themselves they check the quality of the signals being read from the disk platters on each Read operation. If the signal is weak it goes through some routines to read the data off accurately, then writes that to a new spare good sector and replaces the weak one with the good one. This all happens in the background and the OS (Windows) never even knows about it. BUT the HDD keeps track of this activity in its SMART monitoring system. If a large number of such errors and replacements have occurred, the SMART system sets an alarm signal that can be read by your BIOS, which relays the warning to you. The message means two things: (1) this has already happened enough to cause a worry that it will get worse soon; and, (2) the stock of spare good sectors is getting low, so the disk cannot keep on doing this forever. Either way, there is a danger that the system will not keep working, and you WILL start to lose data.

The proper solution is to buy a new replacement HDD and Clone your old one to the new one while you still have all of your data intact - BEFORE the predicted failure. Then you replace the old disk with the new one, and discard the old HDD. I had to do this recently. By the way, I found that, during the cloning operation, it kept finding unreadable sectors and asking what to do. Eventually I just told it to ignore those problems and keep on cloning everything it could until finished, hoping that the unreadable sectors were not actually in use for files. Tests afterwards appear to indicate that was what happened - the clone on the new HDD is working perfectly with no apparent damaged files.

Nah, the message is definitely from Windows. (Inside Windows, while I'm using the PC. It even has a button saying "Backup now" and clicking that starts the Windows backup tool.)

The thing is, I have no reallocated sectors or reallocation pending sectors at all in this drive. SMART error count for both are 0. Which is why I'm wondering why this thing keeps on coming.

I've gotten this message on HDD's that the drive head was wearing out and began grinding on the platter before long. Get a new HDD before it just stops working all together. When i got this message from windows I began getting failed writes and eventually a door stop  

Aw, damn. Shouldn't such a damage cause bad sectors to be recorded on SMART, though? Anyway, I'll look into a new one. Stupidly high HDD prices these days, damn...  

I don't think I had smart enabled not a big deal for me since i knew the drive was bad just had it hold downloads and junk files i didn't need to keep, do you have it enabled in BIOS?

SMART is enabled by default, I believe. I didn't try to disable it.