PC slow - don't know why

Yesterday my PC was fine. Worked perfectly. Switched it of and went to bed.

Today there's something horribly wrong. It takes longer to boot up And then it hangs while doing the simplest task. Eg: open a folder then a subfolder. Normally thats two clicks and its there. Now it takes 3-4 minutes just to open a folder and display its content. Starting task manager took 3 minutes and in it I noticed that the System Idle Process is using 99% of the CPU. Rebooting hasn't help. Takes a long time to shutdown and a long time to start up again.

Again, yesterday it was fine. Today it isn't. Nothing was changed.

Please help.

Just now, after reboot 6 or 7, I get this message during boot-up. Not sure if its related to this...

Have you seen this before? Could it be serious or just some glitch that'll magically come right? (hoping for the latter... )

Thanks.

What caused this though? Trying to understand. Things is, if its the C: drive then easiest thing to do is just reinstall the OS and software. No biggie. If its the D: then its more of a problem as that's where I store all my data... work files, personal files, etc. I am more concerned about not losing that.

Or is a hardware problem i.e. the disk (or whatever) is just failing and needs to be replaced?

UPDATE:

Unplugged the D: drive (storage) and started the PC... ran beautifully. Plugged D: drive back in and we're to the stone age. Clearly the D: drive is the problem. Here's my question: what's causing this? Is it a hardware problem or has something simply become corrupted on the drive? I ask as I cannot copy anything large off the drive, it fails.

I really want to recover what's on there. 8 years of family photos along with various work files... can't afford to lose it. Will the macrium route even work considering I can't even copy stuff onto a flash drive?

Cheers.

Another question:

I want to plug the D: drive into a different PC and test it there. Should I use a USB connector (have an adapter for USB to HDD) or connect it as it was in my PC (don't know the name of connector that in the case, sorry). Would that make a difference?

Cheers

Edit: is this drive saveable? It not even a year old. Would a format fix this?

I wouldn't worry right now about "testing" it or understanding what's wrong.

I'd worry about trying to save whatever is on it as you say that stuff if very important.

I'd first try to make an image of that drive as it now is without any further attempts at repair or recovering files.

After you have an image of that drive stored on some other drive, then worry about testing it or recovering files.

Been going through the help link s above... is there one that isn't video based? I dn't have the bandwidth to watch videos and would prefer something in text form...

Thanks.