1. Install the Intel RST driver.
2. If you have the product key there is no reason to use SIW.
Use official MS product keys only.
3. You don't need to buy a new OS if you have a legitimate MS key.
If you find the previous ISO file it can be re-used to install.
There is a link to get the official MS ISO, as long as you have an official MS key.
How to Clean Reinstall a Factory OEM Windows 7
4. Try deleting the Real Temp files you have and D/L a new copy, run that to see if you're still having issues.
5. memtest86+ will run until you stop it. No way to set it to stop, if you don't have 7 full passes by the time you go to bed then let it continue over night.
The time to complete 7 full passes will depend on your system and how much RAM you have installed. Yours should be 4-6 hours approximately.
If your issues are software related a Clean Install will help, a fresh installation will also run faster/better.
If your issues are due to hardware then the Clean install won't fix it.
Your choice, I would recommend a Clean Install just to clear up some of the issues you are having.
Let us know the memtest86+ results.