Yes unfortunatley, you will have to reinstall windows to get SSE3 support. It's just like switching from a Pentium 2 to a P3 back in the day... to get the SSE support the P3 had, you had to reinstall Windows. You will have to probably do a reformat, as a repair just rewrites your Windows directory.
I am surprised that your PC didn't blue screen on bootup... that's what usually happens when you add something new like that.