All of those things!
Don't wipe and reinstall. That's a sledgehammer approach.
First thing I'd do is just to press Ctrl-Alt-Del when it's up and running, select Task Manager, Processes, then click on the CPU column until it shows running programs in the order of how much CPU they are using.
If nothing is running, but a large proportion of the CPU capacity is being used, look at the stuff using it. That often reveals a lot.
Then look to see if Drive Indexing is turned on - My Computer - click on Drive C - right-click - Properties - General an dlook to see if "Allow Indexing Service..." is turned on. If so, turn it off.
Then, check the stuff like free disk space, defragging, temporary files, and that stuff.