Cooks Illustrated has a method for a supermarket ham that is already fully cooked, to wit using one of those oven roasting bags, which really does keep the ham from drying out. You can remove it at the end if you want to glaze the ham.

Oh, my darling pig, how do I love thee? Let me count the ways: There are fresh hams and fully cooked hams, do not confuse one with the other! Nor should you confuse smoked hams or cured hams with the fully-cooked variety (both require cooking). Dry cured hams (country hams, prosciutto and others) contain so little water (and so much salt) that bacteria do not survive.

Fully cooked hams bearing the USDA seal should be heated to no less than 140F, non-inspected hams to 165F, as measured with an instant read thermometer. Refrigerate or freeze leftovers within 5 days. Serve cold or reheat to 165F.

http://www.fsis.usda.gov/FACTSheets/Safe_Minimum_Internal_Temperature_Chart/index.asp

If it is the basic hams sold today by most companies it has been pumped with water, salt, sugar, and probably sodium nitrite, sodium phosphate and sodium erythorbate. These hams have reached a temperature during processing that kill trichinia and other pathogens, at least 154 degrees F by USDA regulations. The hams are cooked. This goes with boneless or bone-in. The standard reheating procedure is to put about 1/2" water in a pan and heat the ham, covered, in a 325 oven for 20 minutes per lb. The last 1/2 hour, uncover the ham and glaze to ones preference at a 400 oven. My family use to produce York style hams (somewhat similar to today's style ham, except better) and this was the standard instructions we gave all customers for over 50 years. No one ever complained.

And what type of ham?

And how are you preparing it?

Hi -- what size is the ham?