Thanks to all of you.
As you said Kratch, my move tool was not oriented to world. So I could easily solve that problem.
Yes, maybe I should get a better video card as time pases by and I use more fequently the program.
I was also thinking that there must be a reason why maya 2008 is recommended for Windows Vista Business, so possibly not everything is working so fine on my machine, using Windows Vista Home Professional.