Without a reboot, it proably cannot fully hook into Windows. Even stopping/starting services may not be enough, as I would think some parts have to hook in very early in the boot process to give the maximum protection. If a process is is running at a very low level, it probably cannot stop it to replace the file.
Most functionaltiy you need will work without need for reboot, local real-time scanning works, Acces protection and buffer overflow work, on-demand scans work, scriptscan and email scanning work. That should cover your immediate areas of concern.
One quirk you might notice (but really unlikely) is if you scan remote DFS shares with the real-time scanner - you should reboot first for that to work.
And if you're having to apply any patch updates on top of the installation, it's in your best interest to reboot to avoid potential stability issues from the older code that your patch update is trying to solve for you.