A G4400 should be more than capable to download something without breaking everything.....something funny is going on. I'd run a stress test on the CPU, RAM, and on the storage to make sure there are no issues there. Then I'd run an antivirus and anti malware. I'd also try a wired keyboard/mouse. And if that turns up negative, then I'd reinstall windows and make sure everything (including drivers) was installed correctly.
Also, why are you using an HD5450? The G4400's iGPU is good enough for what you're doing (unless you're running more than two monitors).