Let me take a moment to thank you for posting on the HP Support Forums.
I understand that you have an HP TouchSmart 300 series PC. I read that an application keeps running known as "My Presentation Font Cache application" and takes up 50% of the CPU. You have to go to the Task Manager and end the task to that computer works normally. You've done a remarkable job performing the steps. Not worry, I'll do my best to help you.
To provide an accurate resolution, I need a few more details:
Please let me know the exact product model no. This link will help you to find the model no: http://hp.care/2bygf7l
Did you ever install the "My Presentation Font Cache" application and do you use it?
Meanwhile, I recommend you to perform the following steps to isolate the issue and arrive at a fix: