My HTPC has a fairly long boot time. I discovered it was the (2) Hauppauge HVR-2250 Dual ClearQAM Tuner Card's driver initializing that was the cause of it. I used the Windows Performance Toolkit to figure it out. There isn't anything I can do about my boot time because those cards just take a long time to initialize, but at least I know WHY the boot time is slow. Also, the machine is rarely rebooted. I'm running windows 7, but the process is the same for Vista.
Here is a very good article explaining what to do:
There is a link to download the WPT within the article.
Please feel free to ask questions.