You are running a current OS and such on a pretty dated machine. Just the usual indexing and things Windows does will result in CPU hanging around 50% for long periods. Now I'm not at your PC so I can't dive into it to figure which of the WHAT IT SHOULD BE DOING AND IS NOT A BUG THING IT IS THIS TIME but for example that .NET update and re-compile can take folk over the edge hunting for the CPU use for weeks. And that could happen every time .NET updates. A fast machine with loads of RAM helps but this machine doesn't have that.
Also, it could be the old slow possibly failing HDD. But nothing in your post screams that something is wrong. Just that it is dated and if it's the usual, the .net compile or indexing may finish over the next few weeks.
I had to clean a browser hijacker (trovi) off of my system a few days ago after downloading video editing sw from download.com (! - thought it could be trusted). Cleaned the system with Superantispyware and Malwarebytes in Safe Mode (Avira AV didn't see it). Since then, my system has run kind of slow, but last night it was getting bad, so I looked at task manager which showed CPU utilization in the 60-70%'s with no applications running. The Processes tab didn't show anything taking more than 1% including "processes from all users." The crazy thing is that it shows 'System idle process' at 75% and cpu utilization at 70 - huh? Is system idle not that idle?
So, I tried booting in Safe Mode to avoid the startup programs and the CPU was still running at 26%, which makes me think something is up. I have run Avira again and got nothing. Before the browser hijacker, my cpu would run 1-2% after startup.
My system is a hp dc7700 ultra slim with e6300 / 1.86GHZ C2D and 3GB RAM on Win 7 64-bit.
Thanks for any help you can suggest.