How cold is too cold?

Some of the capacitors and the LCD have liquids in them that can freeze at very low temps.

The real issue is trying to use your system at such temps. The hard disk platters shrink when they are chilled (Aluminum) so tracking, data reads & writes can encounter problems. If you do write any data on the disks you could encounter a non-readable disk when you try accessing it after it has warmed.

Lastly, if you don't let the system warm up carefully in a warm building condensation can build up inside.

Best to allow your system a good three-four hours or more to warm up sealed in a plastic bag before entering inside.