What exactly do you mean by, "doesn't understand what I'm sending?"

Does it receive anything at all?
If so, what does it receive?
What do you expect your micrcontroller to make of 0xAA? (It's not a standard 7-bit ASCII code).

Is your PC, VB, etc set up to match your microcontroller's setup; ie,
* Baud Rate;
* Word size;
* Parity;
* Stop Bits.