If you upgraded to windows 10 check your task manager under the process Tab.
If your running a spinning Hard Drive Windows 10 will have them at 90 to 100% load constantly.
I tried all kinds of things to stop this on my PC.
Eventually had to upgrade to an SSD.
As for your overheating check the thermal compound under your APU it may need replacing. If its dry and crumbly or of a putty like consistency it needs to be cleaned and new thermal compound reapplied.
The APU cooler may not be seated evenly if it is working right. May just need to be reseated.
Then also like you say the PC is 9 years old now and may just be severely outdated and overloaded.
Try reducing your screen resolution to 1024x768 or 1366x768 (Go with what your monitor/tv will support in this resolution range) with a refresh of no more than 60Hz or 30Hz. Doing this can reduce the load on your APU and reduce temps.