Software update; can I restart while configuring installation?

I upgraded an older laptop to Leopard (after trying to go right to Snow Leopard unsuccessfully). After I was able to get everything working fine, I went to software update and started the upgrade from 10.5 to 10.5.8. The installation has now stalled. Can I shut off the laptop or will this create system problems? This is what happened when I tried to go to 10.6 and I was not able to restart the computer. Thanks for your help.