My dad's will wasn't notarized. Is it valid?

A will doesn't need to be notarized to be valid, so don't worry about the lack of a notary's stamp on your dad's will. Generally, to be valid, a will must be bear the signature of the person who made it (the testator, in legal lingo) and of two witnesses who watched the will-maker sign. Many states also allow "holographic wills," which aren't witnesssed, but are written and signed entirely in the handwriting of the will-maker.

It's common for the witnesses to sign another statement, stating that to the best of their knowledge the will-maker was signing his will, was of sound mind, and wasn't under any undue influence from anyone. The signatures on this statement (which is usually in a separate document called a self-proving affidavit, but sometimes incorporated into the will itself) are notarized. This makes the will "self-proving"--that is, after the will-maker's death, the probate court will accept the will as valid without making the witnesses testify about watching the will being signed. But a self-proving affidavit isn't required for the will to be valid.