As I was reading your post I was suspecting that the PSU may not be adequate, so I agree with Dai that it's probably not good enough for what you're running.
There may also be heat issues if the fans are not running at full capacity or if they are dusty, etc.
There may also have been some software issues like malware, disk errors (caused by the failed download / install) as well as a botched defrag (sometimes freebies will move the wrong files (probably what happened with iTunes) and sometimes they corrupt system files.
The re-install should have handled all the software issues, but depending on whether you did a clean re-install after a re-format or an "in-place" re-install it may still have some lingering software issues.
I would first address the hardware angle as per Dai's advice and also make sure the fans are running well and the CPU is free of dust (as well as its heat sink, etc.) and that you have enough
fans to keep the system as cool as possible (even though the temps seem to be OK, the temp sensor may be off, etc.)
I had a user test his BSOD-prone, "hopeless-case" PC for heat by taking off the case covers and putting the PC in front of the AC while it was running -- no more BSOD!
Once all the hardware issues are remedied, try starting it with the covers off and with cold AC air blowing on it as above. If it now works fine, you need to replace or add more fans to cool it down properly.
0. First since you already did a re-install, I assume the files you wanted to keep were already backed, up. If so, and if the HDD checks out OK, reformat it and then
1. Make sure you run only one Anti-Virus program and a good Anti-Malware like Malware Bytes.
2. Get and use CCleaner to keep the registry in good shape.
3. For the level of use you probably have, I would highly recommend you look at a commercial Defrag program as the built-in is really meant for casual users. Check out this Top 10 Review side-by-side comparison of the best defrag programs around:
Disk Defragmenter Software Review 2011 | Defrag Software | Disk Defrag - TopTenREVIEWS
The top placers are all automatic and the gold medalist is completely transparent during defrag
according to the review (they also stated that it prevents fragmentation).
Get a free trial and try it out (the top programs are designed to be used in large IT sites -- they don't move files that should not be moved and they don't screw up system files, etc.)
Good luck to you!
Bill R Tech-Spec
PS: Don't be depressed. This kind of stuff happens to the best of us -- it's a learning process. At least some of your hardware (if not all) is probably still under warranty....