Vista Networking Workaround
From KVM
Vista has no ne2000 driver, and the rtl8139 driver does not work with the qemu emulated device.
To get Vista to network correctly, specify the ne2k_pci device on the qemu command line:
qemu -net nic,model=ne2k_pci
and use the Windows 2000 driver disk, available here.
Note: I found that the rtl8139 driver worked with kvm and Vista just fine. I also found the download link above to be broken. So you might also try:
qemu -net nic,model=rtl8139
Note: that's because the emulated rtl8139 was fixed with the last QEMU CVS merge. Vista guests should use rtl8139, as Vista's rtl8139 driver now works.