How can I use my good old mouse?
You'll want a PS/2-to-USB adapter.
They are a very inexpensive and easy solution.
For a bit more of a complete answer - a lot of modern mice and keyboards have processors that can handle both the PS/2 and USB protocols. In these cases a passive adaptor that simply changes the physical connection will do.
There's two scenarios where you need an active connector - firstly, where power draw is significantly higher than expected - some old Model Ms fall under this category. The second is where the mouse is old enough that it dosen't actually speak USB.
Interestingly the famous blue cube I recommend when people are looking for such an adaptor may not work for mice - you'll want an active converter with a mouse specific port - which often means a USB -> 2x PS/2 converter.