I just want to start off by saying that Avast 7 is what is causing the slow start-up problem that many people are reporting. Like so many others in this thread, yes, I have already spent way more time than anyone should have to troubleshooting this problem including multiple clean installs. I have, however, found what the exact issue is within Avast that causes the slow start-up.
The Cause of the Problem:
(short version)
It's the File System Real-Time Shield, more specifically the "Scan programs when executing" option in its expert settings. Which means when windows first starts up and is launching all of the start-up programs that Avast feels the need to, and Win7 x64 seems to totally okay with letting it, scan every single one of them, hog a bunch of the HDD I/O in the process, and create a huge bottleneck system.
(long version)
The reason for the 2 minute wait at the welcome screen and the following 3 minute wait on the desktop is because as soon as you submit you password to log into your computer the winlogin.exe process is started, along with several others that are needed to get the desktop environment and other system stuff going. Avast is apparently already running at this point and the 2 minute wait is that time it is taking it to scan all of those processes. It then takes avast the next 3 minute wait to scan all of the others process that run at start-up. It scans these because of the "Scan programs when executing" option in the expert settings of the File System Real-time Shield. The long wait is because Avast wants to scan these process the best it can according to the heuristics and settings it has assigned to the File System Real-Time Shield and to meet this demand, it takes a bunch of the disk I/O. This means that Avast can scan more quickly, but as more disk I/O is taken by Avast, the slower processes get read from the disk and executed.
The Solution:
Well, short of turning the option off every time you shut the computer down and then back on after you start it up or just dealing with the slow start-up, I don't really have one. Sorry. While I'd love to say that it's Win7 x64's fault for not having the OS better handle this kind of HDD hogging, it really is the fault of Avast. I'm hoping, for their sake, that Avast's silence about the issue is because they are working on it and just haven't gotten to a point where they are ready to make a statement about it, and not because they're ignoring it.
Note: Please, if you are booting off of a SSD (HDD RAID setups may also apply to this case), don't post a reply saying that your computer isn't having any slow start-up issues. When booting from a SSD, at least in my experience, this issue is hardly noticeable since, well, SSD's have way higher transfer rates. And if you are running Win7 x64 off of a non-RAID HDD setup and you aren't having slow start-up issues, it would be very helpful if you gave your system specs and/or a list of either common software that's not installed on your computer or uncommon software that is installed, as this will help narrow down what Avast is conflicting with.