Guaranteed to be (statistically) unique:
string fileName = System.IO.Path.GetTempPath() + Guid.NewGuid().ToString() + ".csv";
(To quote from the wiki article on the probabilty of a collision:
...one's annual risk of being hit by a meteorite is estimated to be one chance in 17 billion , that means the probability is about 0.00000000006 (6 × 10−11), equivalent to the odds of creating a few tens of trillions of UUIDs in a year and having one duplicate. In other words, only after generating 1 billion UUIDs every second for the next 100 years, the probability of creating just one duplicate would be about 50%. The probability of one duplicate would be about 50% if every person on earth owns 600 million UUIDs
EDIT: Please also see JaredPar's comments.