The correct solution is to backup your data, repartition using the correct type, reinstall clean, then restore your data. This is the official Microsoft supported method.
However, there are several completely unsupported, and potentially lethal to your data, methods developed by others. Here's just one: http://mypkb.wordpress.com/2007/03/28/how-to-non-destructively-convert-dynamic-disks-to-basic-disks/
Keep in mind that I'm not suggesting that you do it this way and take absolutely no responsibility for loss of data.