I once tried to tan a prong horn antelope hide with only limited success. As Mike has pointed out, tanning a cow hide would be a fairly involved process (and neighbours might complain of the smell). Since you are getting the hide for free, you might want to find a local tannery and ask what they'd charge to tan the hide for you. I doubt it would be more than a couple of quid, and you'd end up with an excellent piece of leather to work with... unlike my "self disintegrating" antelope hide.