Keep Antivirus Starting Up
You want this protection. After you have eliminated several other processes and your computer is still slow starting up, it could be the fact that your antivirus is using too many resources (memory). We'll cover that later though.
Use Common Sense
Just read it and you'll most likely know you need it. Does it say something about audio? That could be a driver or program your computer uses to enhance audio function. If it's a laptop, there will be a driver starting up for your touch pad, which you also need. Also, anything that refers to Intel or AMD (processors) should also be left alone. If there are any updaters starting up and they aren't causing problems, leave them – this saves you time and hassle. This doesn't necessarily apply to people who know what they're doing, but if you're still getting comfortable you should use caution.
With all that said, there are a lot of good programs that place themselves in the startup. Some give you the option to not do this during the initial installation of the program, which is why I always recommend doing the custom installation instead of the "recommended" one. It's not really any more advanced and it lets you know what's getting put on your computer.
I often see programs such as iTunes, QuickTime or other Apple services starting up. Most of the time these are unneeded and I haven't found one person who intentionally has them start up. Uncheck them. Other common one is Windows Live Messenger, Skype or other messaging clients. You probably don't need to talk right away to people when you start up and if there is an occasion that calls for that, it's actually more productive to have a faster startup time and then click the program you want. This applies to almost everything. For example, I use Evernote a lot, but I don't have it starting up with my computer because I simply don't always use it right away. However, I do want SugarSync starting up because I always want that running. Make sense? If you know you always want it running, have it start up, otherwise remove it.
Use Your Assets
There are some very good programs that can help in speeding up your computer's start up time or just overall speed in general. One way is to declutter your unused programs. I recommend the free version of Revo Uninstaller for this. It cleans deep and removes the programs registry files along with it. Defragging is another helpful tasks. There are programs such as Defraggler and Auslogics Disk Defrag that have the ability to schedule defragmenting. Ccleaner is another excellent tool, like Revo Uninstaller it removes unneeded or corrupt registry files. It also uninstalls software, however that's not what I use it for. I use it as an extra swipe through to eliminate registry files and also to delete temporary files from the web that aren't needed.
Hopefully some of these tips and ideas can help you speed up your computer and become more productive and efficient with the work you do on it. Have any suggestions that I missed? Let me know below in the comments!