GPT stands for “GUID Partition Table” and it refers to a relatively new format for disk partition tables. It was designed to get around the limitations of the MBR format, namely that you can have more than 4 primary partitions and partitions can be more than 2.2 terabytes in size. GPT is part of the EFI standard but it can also be used on standard BIOS only machines i.e. most non-MAC PCs. Converting to GPT will allow you to future proof your partitions. 2 TB disks are becoming common and it won’t be long before the 2.2TB limit of MBR will stop people using all their disk space in one partition, so in the name of usability and flexibility GPT is the way forward.
You have a Windows machine which will not boot up but you can still access the disk, even though it makes various clunking and thunking noises.
Install a new hard disk and Partedmagicos on a USB stick or CD and run either “ntfsclone” (the easiest and quickest option) or “dd_rescue”. If the NTFS structure is damaged and you cannot repair it fully using the windows recovery console the latter option is the one you want. Of course you may have Windows installed on a FAT32 partition in which case use dd_rescue.