Conflicts on my IRQ on Windows 98SE

I have installed 2 USB ports onto my computer (through a PCI card to enable use of a Web cam and Digital camera as my computer is alittle old and had no USB ports).Now I have a conflict against my Com Ports 1 and 2. The conflict is within IRQ 3 and 4. I have gone into Device manager etc and tried to manually change my USB settings from the conflict 3 and 4 to an available number but it is not allowing me to do this. Every time I try it comes up with the message ‘Unable to modify setting’Is there a way of manually doing this? My free available IRQ numbers are 10 and 16 so when and if I am able to change this would these be good numbers to change to? Please help