CPU is 100% during backup?

Im doing image backups over my home LAN 100baseT to a Dell Dimension P3 600
XP SP2, 640mb, to USB 2.0 PCI card to 250G external harddrive.

During this backup time, CPU utilization on the P3 Server is 100% and
network is 20%.

If I do the identical backup to a installed HD on the same P3 PC the CPU
utilization is 75% and network still 20%.

Any way to make USB more efficient CPU use wise.?

What should CPU usage be for something like this?

Any insight appreciated