I wanted to install a wireless mouse on my PC, however it is a bit older and does not have a port for USB or PS/2.

The port where my current mouse is connected to is a 9pin one.

I tried to use an adapter to connect between the 9pin and the USB however the PC did not recognise the new mouse as new hardware.

When I asked in a store they told me that using an adapter would probably not work anyway in this case if the PC does not have the required ports at the back.

Is this correct? Can anyone give me some advice on this matter?

Thank you  

Hmmmm, not sure but alot of boards do have the facility to connect a USB loom to the designated headers on the board.
some are neat front panel brackets that use a spare five and a quarter bay, others are mounted on a expansion bracket and take up one of your upgrade ports at the rear.
sounds like your best bet. They are very cheap, £10, $17 or there abouts.                                    Martin

Try maybe getting an adapter for PS2 instead of USB