Hi vista noob,
The normal way to convert is destructive , i.e. you lose the data, like this
ITsVISTA Tip 32: Convert a Dynamic Disk to a Basic Disk in Vista | ITsVISTA
There is a workaround using diskprobe, but the following method looks more straightforward - haven't used it myself , but looks like it should do the the job - hopefully without losing your data
Convert Dynamic Disk back to Basic Disk without data loss - Wilders Security Forums
Hope it works for you.
SIW2