Avast! Anti-Virus is a robust program that offers a boot-time scan. This is a scan that runs prior to the Operating System fully loading drivers, startup programs, or other places where viruses or trojans can hide from scanners.
With the release of Version 6 there is 64-bit boot-time capability.
I searched the Avast site and could not find a simple link demonstrating how to enable the boot-time scan, so I did a few screen captures and put together this FAQ. 
Note: If you are still using versions 6, 5, or 4 of Avast I have those FAQs available. Links to them are in the Related Links section at the end of this FAQ.
Start the Avast user interface from the Start Menu, by clicking the orange Avast icon in the System Tray, or double clicking the desktop shortcut.
1) In the left column, click the Scan Computer link.
2) Click the Boot-time Scan link.
3) If you are going to do this, we'll want to do it right by scanning all hard disks. Click the Settings button.
4) Now select All harddisks
5) Clicking the orange bars on the Heuristics sensitivity will adjust it. If you're having problems to the extent that you want to perform a boot time scan, I recommend setting it to High.
6) Again, why bother if you don't scan for Potentially Unwanted Programs (often those borderline viruses such as malware and spyware programs). And since viruses do hide in .ZIP or other types of compressed/archived types of files, so be sure to also check the second box to scan inside of them.
7) By default the action taken when a threat is found is to Ask. This can become time consuming if there are many finds, but personally I like having the options depending what is found. Adjust according to your needs. Click OK
8) Once clicking on the Schedule Now all you need to do is a system restart. If anything is found during the boot scan the prompts are self explanatory, read them and follow their advice.
This FAQ was created on a 64-bit Windows 7 system using the Snipping Tool for screen captures.
Installation Tips: If connected to the Internet with anything but a Dial-up connection, be sure to get into Settings/Updates and select both Automatic Update settings as well as "My computer is permanently connected to the Internet." I also go to Sounds and uncheck the sound for Automatic Update. It can be a little unnerving when nobody is using the system yet it starts talking! Beginning in Version 5, sounds are no longer profile specific, so any user account (and it need not have administrator rights) can change sound settings for all accounts.
 I'd like to thank all of of the forum members of the Avast support forums for making so many references to this FAQ. Thanks!
Disclaimer: Schmahl World Computer Assistance is now a reseller of Avast software. To inquire about purchasing any of Avast's products please visit the contact page for ways to get in touch for licensing or other questions about Avast products.
http://www.schmahl.net/avast6bootscan.php - instructions for a boot time scan with Avast version 6
http://www.schmahl.net/avast5bootscan.php - instructions for a boot time scan with Avast version 5
http://www.schmahl.net/avast4bootscan.php - instructions for a boot time scan with Avast version 4 (c'mon, upgrade, will ya!)
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Schmahl World Computer Assistance, LLC
this page: http://www.schmahl.net/avastbootscan.php
updated February 25, 2012