Maybe you don't exactly need a serial to parallel converter.
some uC's like the at89c2051 have a TXD and RXD or similar serial transmit and receive ports.
Just set the clock to match the speed of the serial port transmission. and don't forget about the start and stop bits.
When using a microcontroller, you need to remember that another program is executed each time the uC is turned on or reset, and the speed of it is determined by the clock attached to it.
So first, you need to write a program that manages serial port data and then send it to the microcontroller. then write another program that manages the serial port data, but this time, on the computer.
hook everything up, and you will be ok.