Slow boot, even during clean boot

For a couple years now my computer has been booting slowly, no matter what I try to do to fix it. Typical boot time is between 81-92 seconds, and even a clean boot took 77 seconds. I know it's not as bad as it could be, but I'm very proud of how clean my computer is, and it's frustrating.

One thing I've tried is using xbootmgr in the Windows Performance Toolkit and installing hotfixes as mentioned here, but it seems to have an effect only for the first few boots after using it and then it just goes back to the way it was. This has been going on for long enough that I don't remember the specifics of when it started, but it seems to have been around the time I installed Visual Studio 2010 Ultimate, so after some googling I attempted running "ngen.exe executequeueditems" for the .net framework as mentioned here, which took a few hours, but that didn't resolve the issue either.

The majority of the boot time seems to be spent on the "Starting Windows" screen, but the "Welcome" screen takes longer than it should as well.

I'm running Windows 7 x64 SP1 on a custom built computer. The specs, if you need them, are:

ASUS M4A79 Deluxe mobo using onboard sound
AMD Athlon X2 6000+
Geforce 8800GT
6GB DDR2 RAM
1x Western Digital 640GB HDD
1x Western Digital 250GB HDD

Any advice?