HDD suddenly using 499.25gb of 499.52gb after clean install of Yosemite

Hi, my MacBook Pro has been slow and crashing etc. for a while. I did all the usual: repaired disk permissions (which, I noticed, always needed repairing, even straight after fixing), reset SMC, started in safe mode etc...eventually it got so bad I decided to restore from a TimeMachine backup. This took me back to Mavericks, but after a while the problem returned...soo slow, apps crashing and hanging. Then I noticed a strange thing: in Disk Utility, the HDD was apparently full - roughly 499gb used with only 19MB available! I knew this wasn't correct - simply not. I tried all the usual: 'Get info'..OnxY...even another programme I downloaded to find hidden large logs or caches could only find something like 130GB used. So now, fuming, I decided to just erase the disk and reinstall OSX (only Lion available to me in recovery mode). Did that and then tried to re-upgrade to Yosemite. That's when the fun started...attempts to do this failed a number of times (install would hang and fail for various reasons). Then my backup disk wouldn't show in Disk Utiltiy, and after one final attempt to get Yosemite on, it failed half way through again and my whole HDD disappeared. Anyway, after a week of bullsh*t and faffing around it turned up. I re-installed Lion and I made a bootable disk of Yosemite on a USB - erased the HDD again and did a clean install. First attempt failed (could not be verified or something...after waiting about a day)..second time worked. Success! no. After about a day of a pure MacBook and seemingly running ok, it started slowing, hanging and crashing again. Just checked Disk utility and sure enough, 499GB used again. Disk permissions screwed (ages to repair again). All I have done is download Chrome...there is nothing else on the **** thing!

Does this thing have a virus or something? I used to have Cleanmymac2 on there a while ago..but all that must have been deleted. When Mavericks played up I downloaded Avira..took 2 days to scan (the restore from Time Machine) and found nothing...looked for flashback..not found.

So to re-cap. After a clean install of Yosemite from a bootable USB the problem I had is back! disk utility says the HDD is using 499GB. I've not added anything - not even re-added my music (which would only be about 89gb anyway). The crashing, hanging, and general slowness is also back. One last weird thing. The HDD name seemed to change when I first erased it - went back to TOSHIBA then a long number - fine. The partition I called Macintosh HD again. Now the HDD at the top is also called Macintosh HD. Never done that before. Also (I'll describe what I see), the main HDD (what was TOSHIBA) is at the top of the list in disk utility and the (what I image is the partition as this is where Macintosh HD used to appear) is under it, slightly offset ( a different branch). when I select this one, it only say 12GB is used....!?!? (which still seems high as there's nothing on it, but nothing like 500gb!!)

also, in monitor CPU usage is non-existent. Problem doesn't appear to be there.

So over it. Any suggestions would be much appreciated. Google searches don't offer much.

Thanks everyone, and go easy on me...I'm no expert and know very little about all this.

mac is MacBook Pro (13-inch mid 2012)