problem with usb ports

[jimmyp]

I installed Kubuntu last month and I have a problem with the usb ports. When I connect a usb device such as a modem or a printer my system crashes. The same happened when I had installed Ubuntu. My system:
motherboard MSI 645E Max series Sis 645DX Chipset based
Intel pentium 4 2.4 Ghz processor
512MB RAM
2 hard disks WD 80GB and 250 Gb

Please help me!