No. A few years warning is absolutely necessary to deflect a large asteroid headed for Earth. There are no technologies on Earth that could possibly deflect a large asteroid in a couple of weeks.
That depends on what you mean by "large asteroid". Anything circa 100m can be deflected by planting nukes on one side, or even by using daft measures like painting one side white. It is not difficult to deflect an asteroid of this size in outer space.
If you're talking the really huge "planet killing" types, we already know where they are and none are likely to hit us in the next 100 years unless something goes incredibly wrong, eg two of them bash into each other and sends one towards us.
Part 1 - http://www.youtube.com/watch?v=vQp7cVu3Ny0
Part 2 - http://www.youtube.com/watch?v=Ano2y1ewwzQ
Part 3 - http://www.youtube.com/watch?v=Nmi--Ov_pSs
Part 4 - http://www.youtube.com/watch?v=gdu8qAMG_ow
Part 5 - http://www.youtube.com/watch?v=vO62ibV1ojU
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