@ dve83,
How often do you clean up your machine? Many of us here use
CCleaner, a
freeware system optimization, privacy and cleaning tool. There is a
Slim version available as well at
http://www.piriform.com/ccleaner/builds - 4th option down. It removes unused files (cache, temporary Internet files, etc.) from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. Additionally it contains a fully featured registry cleaner (I suggest making a backup as a "just in case in My Documents").
Do you know what software was running prior to shutting down (check Task Manager)?
Depending on your OS, you may also want to defrag it once in a while. There are many good free defrag tools available and you will get many opinions on what to use. I use free Puran, which also offers a boot-time defrag
http://www.puransoftware.com/Puran-Defrag.html.
If you made changes to your system prior to shutting down, your machine will also be setting those changes and depending on your hardware it make take longer than with others. These are just a few suggestions. This does not sound like an Avast problem.