Every time my computer started up & stayed at 100% CPU usage it was because of driver conflicts!
So something you installed has some drivers that don't agree with Windows 7 at all!

Start uninstalling programs that you installed last & see if it stops & what program is causing it.
OR if you have system restore still enabled, try last known good cfg or restore to an earlier point & see if that fixes it.
If it does, then see what program causes this by installing them one at a time & reboot after each installation until the problem returns...then you'll know what it is & not to install it or look for a recent program patch for it.