I used to get rather frustrated with the length of time that AVG took with the virus check at boot-up. Then I realised that (as long as AVG has been doing its job anyway), it's completely unnecessary. So I suggest going into AVG's settings and disabling the scan at boot-up.
However, I'm still using Widows ME and AVG no longer supports my antiquated operating system. So I've made the change to using Avast! I find that it does everything far quicker than AVG does. (Lots of other ABers have also switched to Avast!, after reports of problems with AVG8). So perhaps it's time for you to ditch AVG altogether?
http://www.avast.com/
To address your actual question directly though, it's perfectly safe to leave your PC switched. When I worked in the rail industry, our PCs were booted up on the morning of 27 December each year and not turned off until the evening of 24 December the following year. They never came to any harm.
However, to reduce your electricity bill (and to be kind to the environment), you should consider using 'Hibernate' (which uses the least amount of electricity), 'Sleep' or 'Hybrid Sleep' (which are only available in Vista). The differences are explained here:
http://windowshelp.microsoft.com/Windows/en-US /Help/335c6a5d-0304-4af1-b135-6bf6c124dc111033 .mspx
If you're using XP, the 'Hibernate' option is hidden. To enable it, follow these instructions:
http://www.microsoft.com/windowsxp/using/setup /learnmore/tips/magyar1.mspx
Chris