Complete Freezing On Two Computers

I have two ASUS computers: x83v (notebook) that is about 4 years old & 1015PEM (netbook) that is about 1.5 years old (still a baby), both of which are running Windows 7 Ultimate (64- and 32-bit, respectively). The freeze I get seems like it is the operating system since I still have mouse control for a few moments, but I am unable to open any programs nor the task manager; shortly after, the mouse goes down too (the mouse freezes with everything else). The time duration before my computers begin to freeze vary: my notebook is usually after 10 minutes, and my netbook is almost immediately. I began experiencing the complete freeze about 1.5 months ago on both machines. Being a tech-savvy guy, I ruled out hardware issues since both machines experienced the same issues at the same time. I used my netbook only for college, so it didn't have much on it. I began comparing similarities on the two machines: Dropbox, Avast, & Office 2012 Pro+. I uninstalled Dropbox and nothing was fixed. I did not test removing Avast or Office. I decided to back up my important files and format and reinstall Windows 7 on both machines. I imaged them with Acronis, when they only had updates and office 2012 installed (no virus scan). After the imaging process, I added Avast and Dropbox again, as well as Code Blocks for C++ coding (on my notebook); I did not restore my important system files yet since I am trying to zero in on the issue. I am still experiencing the freezing on both machines. Very seldom my netbook will recover from a freeze. Last week, my notebook needed Windows updates so I started them and went to sleep. The next evening, nearly 24 hours later, I noticed my computer was still on: it froze on update 3 of however many. I was very reluctant to shut down the computer in fear of corrupting system files, but I knew it was not going to recover from the freeze since it had been 24 hours already. My brother is 12 and downloads random things frequently and is not very careful. I wondered if he caught a bug and it had spread to my computers. I tried multiple other virus scans on my computers, including McAfee Stinger & Malware Bytes, and everything came back clear.