Scheduling antivirus checks and updates is something that needs to be done daily. People that use their computer for business or other serious applications need to schedule three things to occur overnight.

One, update the antivirus definitions from the software manufacturer. Two, scan the disk for problems. Three, defragment the disk using windows or some other defrag tool. Then the computer is in top operational performance when morning rolls around and you need to get going.

• Update definitions as often as reasonable, but not more often. If antivirus definitions are released at a certain time each day, it's completely pointless and wastes everyone's time if you've scheduled yours to update hourly. The recommended time of day is almost always the best time, don't try to outguess the software on that. Even if it does it while you're using the computer, it shouldn't be noticable on anything but dialup.
• Schedule a full disk scan to run while you're asleep. Feel free to reschedule this if you're usually using the computer at that time. We recommend setting it to scan every file, not just executables, and scan inside compressed files. In fact, turn on every single option you can, including the heuristic scanner to maximum intelligence, because you don't care how much CPU it takes in the middle of night.
• If you don't want to leave your computer on all the time, schedule it for every night, but then just turn your computer off every day except Friday. There's no point in trying to schedule things around when you might leave your computer on. If it's on, it might as well scan, and if it's not, it obviously won't.
• Be sure to schedule the full disk scan after the definitition update. A small thing, but you'd be surprised how many people's computer scan for viruses at eleven at night, and update their definitions at one in the morning. That's not very logical. Some antivirus lets you schedule them in one step, with the full scan starting as soon as it has new definitions.