You need to chose to format the disk completely using standard options to get a MBR rather GPT disk. Obviously you will lose all data. GPT isn't necessary for a 500GByte hard-drive.
To convert the disk back try this link, although you will need to have a working system you can plug the drive into: http://www.sevenforums.com/tutorials/26203-convert-gpt-disk-mbr-disk.html
You also might be able to do this from the Windows 7 install disk using F10 and Diskpart, a Google should find you some useful info.
Hope that helps.
I currently have Windows XP pro 64 bit running on an existing disk. I am formatting a second disk to run Win7 Ulitimate 64 bit. I recently found out there is no version of ITunes which will run under WinXP Pro 64bit which is the only convoluted way to back up an IPhone.
I am formatting the new disk in MBR NTFS... Thanks so much for explaining this! It is greatly appreciated.
Donald R. Resor