Question on Hardrives?

After we updated our computer(both are compaq),We just put the old one away,
It still worked fine when we changed them out.My question is can I "clean out" the junk from the old one and use it as a storage drive for our digital photos(way to many in this one),We still have a photo program that retrives photos in the old computer,I would like to use it also as a photo editing and enhancement(crafts and such)it would not be connected to the internet.
Is this possible and if so how would I connect them (usb etc),and would there be a chance of messing up the main computer(DH would kill me lol)
Thank you for all your help.
Pheadra