If you are using Windows XP and want to install the OS onto it; plug the HDD into your computer and turn on your computer with the OS disk in the computer. While the computer is loading the BIOS press F8 and select the disk medium that the OS disk is in. Once in the menu to install the OS in a partition, reformat the drive to NTFS and go through the process of installing an OS.
If you HAVE Windows XP and you want to install the old HDD; first, make sure you have Administrator Privaleges on that computer. If you have those privaleges, go to Start > Settings > Control Panel > Administrative Tools > Computer Management > Storage > Disk Management. From there look for the drive that you just connected to the computer and format it to NTFS. From there you can specify the partition size.