My wireless mouse works, but .........

Hello, I connected a wireless mouse to my pc , and hasb been working good.
Now I need to use the USB port for a portable hardrive, and I tried to reconnect the old wired mouse( that worked
before) and I can't get it to work.
When i discinnect the wireless one I have no mouse.
Can you help me with this?
thanks a million,
I have Windows XP pack 2.

ThaMusic