MBR-based partition table schemes insert the partitioning information in the master boot record (MBR) (which on a BIOS system is also the container for code that begins the process of booting the system). In a GPT, partition table information is stored in the GPT header, but to maintain compatibility, GPT retains the MBR entry as the first sector on the disk followed by a primary partition table header, the actual beginning of a GPT. Now here comes to the problem to convert this disk to MBR disk, there is a very simple and easy method to convert this disk to MBR disk but you will lose your data and partition from that disk. I assume that you have an empty disk where you have accidently enabled GPT and you need to convert it back to MBR disk.
If you are using Windows XP and you are not able to access or modify GPT protective disk, you can easily convert a GPT disk to MBR Disk by using the clean command under DiskPart Disk Utility, which will remove all data and partition structures from a GPT disk.