First of all, GPT and NTFS are things in two different categories.
GPT is a partioning table method
Another partioning table method is MBR (Master Boot Record) or MSDOS
NTFS is a type of filesystem, other types of filesystems are FAT, FAT32, ext2, ext4....
A disk on a modern desktop is usually partitioned in either MBR or GPT.
Those partitions are then formatted with a filesystem, such as FAT, ext2, NTFS, ...
You can install windows 7 by either:
- Using an UEFI equipped motherboard, booting the windows DVD, partitioning the drive with GPT and installing windows on a NTFS partition.
- Using a BIOS equipped motherboard, booting the windows DVD, partitioning the drive with a MBR and installing windows on a NTFS partition.
If you boot with a old fashioned BIOS and try to install windows on a GPT partitioned disk then you will get an error. This is probably what is happening in your case. (An assumption since you did not specify the precise error you get).
You solve that, either wipe the old disk and then reinstall, or change your motherboard to the right boot mode. (Some, not all, support both EUFI and BIOS modes).
To wipe the disk: Boot the windows DVD, press SHIFT and F10 to get a command prompt and use diskpart with the clean option. If you have multiple harddisks then make sure you select the correct one.