Most of the time you work with basic disks, those are the ones we know and love with primary partitions, extended partitions, logical partitions and so on. Dynamic disks are new to Windows 2000 and XP and are formatted differently. Microsoft describes the difference here. Converting a dynamic disc to basic will delete all the data from it! But there is a troubleshooting guide you can follow to try to save the data. Backup first, of course, using a disk imaging program that’s independent of the OS like Norton Ghost, then import the contents of the disc.
What is a Dynamic Disk?