Well, probably in the area of replication, meaning they have to replicate in order to continue their existence. That would be the main one. Viruses are nonliving entities that make their existence by invading other organisms, taking over their cells' reproductive framework and insert their own DNA into the cell's DNA. The cell goes through it's normal reproductive sequence, produces lots more viruses, which then go and invade more cells, and the process starts all over. Oh, and the cell that was initally invaded? Most die.
Protists, on the other hand can be unicellular or multicellular living organisms, capable of performing their own life functions without having to invade another organism. Algae is a good example, able to produce it's own food by photosynthesis. Plankton is another good example of protists, able to exist in the oceans and be a valuable food source to other organisms higher up the food chain, such as the great whales.