I have had similar problems with HDDs a few years ago, and coincidentally, they were also Seagate HDDs. The first HDD I purchased would not format out properly. After much ranting and raving, Spinrite came to the rescue. Although there were no physical errors, Spinrite just would not run through he whole drive. I exchanged the HDD, and this time, ran Spinrite first, and the same problem. The third HDD was OK. My girlfriend PC (purchased at the same time), a few days later, started suffering HDD errors (software related). The drive was exchanged, and it too checked out OK, and both HDDs are still running fine today. Since then I have taken special attention on HDDs in PCs that come through here and have been surprised by some of their condition.
I too have suffered the KAV6 install problem, which made one of my partitions unstable. However to be fair, on subsequent installs it installed flawlessly.
My recipe to solve your problem, assume nothing, test everything. Uninstalling KAV may be a fix for things today, but other problems may surface tomorrow.
Get a copy of Spinrite from www.grc.com, take the HDD out of the faulty PC, plug it into a known good PC and check it. Then methodically check the other components against know good ones.