To me that sounds like one of two problems:
1. You use a buggy version of MySQL or XAMPP, or maybe a combination of both. I use a clean installation of MySQL, Php and Apache, all separate and I have never experienced this problem, I find that combined "noob friendly" solutions may be easy to manage, but usually performs less than desireable.
Please post the version of MySQL if possible, if not please post the version of XAMPP.
2. It might be a bug with either the server, database handling or the database itself that causes a massive amounts of either connections or queries. Download MySQL gui tools and run "MySQL Administrator", then log in and check "Server Connections" and "Health" to see what exactly is causing the massive processor usage.