How are cells and viruses alike?
Cells and viruses are alike in several categorical as well as conceptual ways; meaning, they can be considered in terms of "have/have-not" components and properties, as well as ways that they behave which humans ascribe qualitative value to (such as what we consider to be "alive").
Cells and viruses both use nucleic acids to store their genetic information, and they encode this information in the same way; with specific, compatible nucleotide bases, organized into genes. Both use the same protein production "machinery" in order to transform this information into a metabolic product; specifically, they both use RNA as a "blueprint" for ribosomes to follow when making a protein. Both require energy in order to achieve this; in the case of the virus, it simply uses the cell's energy.
I would consider both cells and viruses to be evidence of life; cells obviously, but viruses because they depend upon things we consider to be "living" in order to complete their own "life" cycles. If we found a virus on Mars, I think most scientists would cautiously consider this to be evidence of life, even if that life wasn't immediately discovered.
They are also alike in that we have no fossil record of their origins, nor the ability to synthetically create them at present. This leaves significant questions about how and when they formed, and to what degree their modern forms and properties are representative of their origins.