Interfacing pc mouse (serial port DB-9)
A serial mouse is designed to connect to a PC's true serial port - it's an actual RS-232 device using ±5V. As such it won't connect directly to the Arduino's RX/TX pins, you'll have to go through an RS-232 transceiver chip.

It also gets its power from the RTS line, but I never knew what the current draw of one of those things was - be careful trying to power it from the Arduino!

Different mouse manufacturers used different protocols. The original standard used a three-byte protocol at 1,200 bps 7N1. The encoding had the following properties:


The leading bit of the first byte of the packet was set - all future bytes had the leading bit cleared.
The first byte had the buttons' states, and the most significant bits of the X and Y deltas;
The X delta was encoded in the second byte;
The Y delta was encoded in the third byte.


  http://paulbourke.net/dataformats/serialmouse/



Other manufacturers added things like extra buttons and scroll wheels that didn't fit in the packet structure. So they modified the protocol, but stuck with some features of the original:


The data rate was (usually) still 1,200 bps, although sometimes they used 7N2 or 8N1 instead.
The leading bit was still used to indicate start of packet;
The X, Y (and Z) wheels still indicated deltas.


  http://www.cpcwiki.eu/index.php/Serial_RS232_Mouse