When second pc is powered internet lost on both

I have the following problem on a friends network after installing CIS5 on both. I don’t know if this is CIS but this did not happen before.

When I power on the Win7 64bit PC he has Internet, when powering the second WIN XP 32bit PC all Internet is lost. When he powers down the XP Internet is back on the Win7.

  • Modem > Router > PC1 W7 with usb (printer) and PC2 XP prints over router over PC1

Does anybody has a clue what goes wrong?