Car starts but wont go

I boughta peugeot 106 automatic two weeks back. I only started driving it last week and until now it has been fine. I went to go off in it yesterday, and it started fine. I started up the road and it wouldn't go faster than 20, so I had pulled in to turn back. On the way back it wouldnt go above 10. I then went to park (rather then abandon it in the middle of the drive) and it wouldnt go at all. It's revving alot, and it reverses a small bit.

I noticed when I first got it, that it wasn't great at reversing when I first went out, but if I left it running for awhile, it was grand.

I rang up a garage and they told me it is either the gearbox (which they told me there is no point fixing as a new gear box is more than I paid for a car) or else the transmission fluid might be low.

any help would be appreciated.

also; do I have any rights as a buyer (i bought privately) with this car, i've had it only 2 weeks. And we really cant afford to be throwing away money if the car is useless.