"the mouse has to be a combination mouse capable of running through USB."
As in, it has to have the circuits for both a PS/2 and a USB connection built into it - a combo mouse always comes with a simple adapter when new to convert it to the other type of port connection - using such an adapter with a PS/2 only mouse will not work.
You can often tell whether the mouse (or keyboard) is a combo mouse (or a combo keyboard) or not by looking at what is printed on it's underside.
If by ps/2 to usb converter you mean a cheap simple adapter you use to connect the PS/2 connector on the cord to USB port use with,
e.g. like this:
http://www.tigerdirect.ca/applicati...
that won't work unless the mouse is a combo mouse.
There are more expensive wiring adapters with included circuits that allow you to connect a regular PS/2 only mouse, or both a PS/2 only mouse and PS/2 only keyboard, to a USB port,
e.g.
http://www.tigerdirect.ca/applicati...
but a cheap new combo PS/2 / USB mouse, or a cheap new USB mouse, is often cheaper.
There are also combo PS/2 / Serial mice or visa versa, but you're unlikely to find a new one these days, and the simple adapter wiring for it varies - if it didn't come with the mouse it may not work.
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Rarely, a USB connected mouse can conflict with other USB devices - I have a Logitech one that does, when some other USB devices are connected.
A PS/2 connected mouse connected to a PS/2 port extremely rarely if ever conflicts with other devices.
....
If you were trying to free up IRQ 12 for use by other devices, that seldom helps.
A legacy PS/2 mouse port uses IRQ 12 exclusively - no other device can use that IRQ if a PS/2 mouse is using it - other devices can use it only if the PS/2 mouse support is disabled (or sometimes, Auto) in your bios Setup, and only if your bios can allow IRQ 12 to be shared with PNP devices, rather than not be availble at all when the mouse support is disabled.
Very few other devices can use IRQ 12 if the PS/2 mouse isn't using it - e.g. some network cards, and USB can use IRQ 12 but you often have to disable USB in the bios, save settings, reboot at least once, then enable it there, save settings, to get the USB to possibly switch to using it.
