The right answer is to leave it on 24/7/365.
I haven't turned mine off in months. The VDU powers down after 30 minutes, and the hard disks are set to something similar. (It's all in Power Options in the Control Panel).
At some ungodly hour of the morning, the machine checks AVG for virus updates, then loads those, then does a full virus scan. Then it does a defrag and a few other things. Then I wake up.
I have a timeswitch on the power lead to the speakers, so they go silent at 10pm (memsahib's bedtime) and come on at 9am. There's probably a way to do that with software, but I had the timeswitch and it was dead easy to plug it in.