I have 2 hard drives in my pc
I need more space so im gonna get another but still want to use one of my old ones

can i stick it in external hard drive enclosure and use it that way?

Yes provided that you get the right one. You need to determine if you have an IDE drive or a sata drive (read how) & you have to get one for the correct size of your hard drive. If it is a desktop you probably have a 3.5". You'll also need to know the hard drive interface type so that you know what to buy.