To get a driver for your mouse, you need to go to the manufactures web site of your mouse and see if they do driver updates (they really should), you will obviously need to know the mouse model and get the latest driver for it.
I have seen many MS patches and security updates have an adverse effect after installation and when you are talking about a patch that supports older OSes like win98 and winME and the later w2k, xp and win2003 server, I personally can't see how one size fits all. There are huge numbers of permutations out there when you are talking about OSes, hardware, drivers, etc. that on occasion when several things coincide a clash can happen. You may also find it difficult to find drivers for your mouse because it is for the win98 OS.
As I said I can't make the decision for you. Your OS is already very long in the tooth and poorly supported (should read not supported) by MS even when it comes to security updates. Because you are running win98, you can't run the latest version of IE6 so these thing already make you somewhat more vulnerable.