If you had a grub.conf file, then you are using Grub Legacy and you should be able to reinstall Grub from the CD you used to install CentOS. Another CD with Grub Legacy should work also. If you know the drive/partition where your boot files are it should not be a problem. You didn't indicate how many drives/partitions you have so if you have your CentOS on sda1 and want Grub in the mbr of sda you could use, from the grub prompt (grub>):
You will need to make the appropriate changes for partition/drive.