thanks dawkins! it's quite interesting actually, i never thought about it until i watched a tv show this morning about mosquitos and the spread of maleria (random i know!)
you cannot catch aids full stop, you can catch HIV from a mosquito, but very unlikely as the virus can only survive in such an organism for a very short time. Thankfully the HIV virus is very fragile and needs a complex organism to sustain it.
As far as I know, the bit of the mosquito that punctures your skin has 2 tubes in it : one to pump an anaesthetic into you so you don't feel it and one to draw blood up. They never mix and nothing that carries HIV ever goes into you.