History will repeat?
If you don't identify what is causing the problem how will you stop it coming back and doing the same after a reinstallation?
Use Task Manager. this will identify the rougue process when the cpu goes to 100%
Also, you can select columns to view that show you the memory used, so again you can identify which one is growing and growing over 2 days.
(As an aside, one of the more popul;ar p2p programs used to have a memory leak and started running at 100%cpu in rimary mode - do you use any of those?)