I use the free Bitdefender anti virus, it's supposed to be one of the best free ones out there. I actually changed from Avast to that a while ago, but I know that one is good too. Doubt your anti virus would be the issue though, but an easy way for you to have checked if any newly installed or updated software caused it would have been to do a system restore to a previous date before any issues had ocurred. I'm no expert, although I actually have had a bad pool header issue myself a few years ago, but from what I understand this can be caused by a variety of hardware or software issues. So it's hard to say what's causing it for you. In my case it was a RAM memory issue, I had to replace one of my RAM sticks. It does seem more likely to me that in your case it's also that and not a hard drive issue. Ofcourse some laptops have RAM integrated on motherboards, so the whole motherboard would then have to be replaced instead of just a RAM stick, but that's not the case for all laptops.
Honestly though, I think you're better off on some tech forum where you can get some better expertise, it's what I would do. At least, after doing the uninstall, system restore, and if that fails format and reinstall of windows. Rule out software issues. And have you considered to bring it to another repair shop? Maybe one that is well reviewed online or someone you know can recommend it to you. As you say you don't have a bootable windows 8 cd, and I imagine you don't want to open the laptop yourself. So there's only so much you can do without expert help.
Are you sure you don't have a warranty on it anymore?
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