Radiator leak

I have a 2008 Aveo and just rolled over to 40,000 miles. Check engine light came on and engine running hot. There was virtually NO coolant in the reservoir and I had an oil change where they supposedly checked it 50 miles prior to this happening. I cannot see any visible leaks though. And after a week I was down about another pint of coolant. Any suggestions?

Is it easy to home replace the radiator or should I have it done by a pro? No air conditioning.

It took me 30mins to make enough space to get the car in the garage and 30 mins to drain the coolant and get the radiator out.
- Drain the coolant (It has a drain plug. Standing in front of the car looking into the engine bay--look to your left and straight down below the top radiator hose. Look along side the radiator; near the bottom on the side is a black plastic plug that is hex shaped (approx. 12 mm) with a cross slot in the head of the hex.
If you put a light near the top radiator hose shining down, you can see the plug when looking thru the lower horizontal slot in the front fascia.)
- remove 2 nuts and 1 bolts to loosen up the powersteering resevoir. This thing is totally in the wrong place for this job and complicates the whole thing.
- there is a hose that goes from the resevoir down to a pipe. This pipe is affixed to the front of the car with a pclip and nut. Remove the nut so you can move the resevoir a little better.
- remove the fan shroud (2 bolts at the top of the radiator, disconnect the electrical connector to the fan, then pull out, The bottom passenger side is held in by a clip. I had to pull pretty hard to get it un stuck.
- disconnect 3 hoses (upper, lower, and a small degas hose?)
- remove the two mounting bolts at the top of the radiator
- at this point I was able to wiggle it out of there by lifting the passenger side up and dropping the drivers side down until it had rotated about 90 degrees. Again, the powersteering fluid resevoir was RIGHT in the way and made taking it out a big pain in the butt. You might take out the battery and see if that gives you more room to push the resevior back.
- the vertical passenger side of the fan shroud had rubbed through the radiator. I used the dremel cut enough of the fan shroud away so that it will not rub through on the new radiator. I am going to take it to a radiator shop today and see if they can just fix it so I don't have to buy a new one.

Let me know how it goes for you.