Mid-2010 27" iMac very slow, even after clean OS install - HELP!

Question

Good afternoon guys, I'm hoping someone can offer me some advice.
I bought my Mid-2010 27" iMac in May-ish 2011. Everything was brilliant, I couldn't believe the ease of use, and especially the start up speed, coming from a world of PCs. My iMac was used almost every day, for web browsing, photo editing, etc.
Maybe about April / May this year it started becoming very slow, slow to boot, slow to respond to mouse clicks, slow to do anything. Since I'd used about 750GB of my HDD (mainly photos), I decided I would archive some and hope that would ease the congestion. As my photos were stored in Aperture, it took about 3 days to export them all to my external HDD, 3 days of the iMac only getting so far, switching itself off, then me finally getting it to come back on (I was getting messages about Kernals, etc. now and again through boot too) - it finally backed up my photos I have from my daughters first year.
Great news, but I still had probably 400GB / 500GB of photos from her second year and any other photos I'd taken. I found the root of my Aperture directory, and managed to pull these images off, so everything is now backed up.
I decided I should try and clean install, and managed to get the HDD to boot and choose the option to re-install from Apple's Servers. All installed OK, and we're good to go! Fired up the iMac, and although it's fractionally quicker to respond, and a lot more stable, it's not quite right. And I've lost all my iLife Apps which originally came with the iMac.
Does this sound like a HDD issue? As I say, it was on quite a lot through the backup procedure, and I had set it (maybe stupidly looking back now) to never sleep, and for the monitor to only switch off after 3 hours. Or does it sound like something more is wrong with it?
I am quite confident of working with PCs, so would manage to replace the HDD I think, looking at some videos, etc. online. I did upgrade the RAM to 16GB, and that was simple enough. My question is, when you replace the HDD, how does it know where to look to the Apple Servers for the clean OS install? Or do I have to do something to let it know where to look first? I have to point out I don't have any Time Machine backup, etc. as the iMac was just not stable enough to get this in place - and up until that point, again probably stupid now looking back, I had never bothered to set up.
Any help / advice on this matter would be greatly appreciated. At the moment I have totally fallen out of love with the iMac, for something to cost £1,500+ and only last 3 years, where my last build PC lasted Approx 8 years for half the price, is very disappointing.
Thanks in advance,
Col

Answer 1

Just suggesting things right off the bat here-
To me it sounds as though the HDD is failing, if your iMac shipped with a Seagate drive try entering your serial number to see if Apple will replace it:http://www.apple.com/au/support/imac-harddrive/
Or try running Apple Hardware Test a few times back to see if it is a hardware error.
As for the iLife apps they should've come on one of the install discs that shipped with your mac.
If your iMac is ineligible for the program above I would consider replacing it yourself with either an SSD or HDD, as for recognising apples servers for download, you will need the original install discs that came with your computer or if you haven't already it will need a firmware update to enable internet recovery
See here:Computers that can be upgraded to use OS X Internet Recovery
Good Luck!