Slow booting can be caused by several things. Usually when it hangs it is a driver or update program that is having problems. For example when adobe reader updates it can take a few minutes to check to see if there is a update and so on. These are annoying as the intent is they pop up first thing to say they have an update instead of doing in the background and showing a message a few minutes after booting.
What can also happen is having to reboot it or force it to reboot it can corupt a setup and the program stalls or won't run the next time.
What you need to do is remove any programs you can out of the startup menu to start with. There are a couple of speed loaders that may be in here. Office and adobe as examples. Just move them out of the startup or delete them. They don't need to run and I've never seen them actually save any time in how fast the programs start.
There are other programs such as MSN or other programs that are set by default to run at startup. Those you can change to start when you start them not when windows does. These also can slow things down bu usually when you see the desktop.
The next place to look is add remove programs. Look for any programs that you have recently installed or do not use anymore. If you can remeber you may be able to spot which one was the one that was installed just before everything slowed down. Again this could be updates or corrupt parts of the program.
At this point if you haven't found te problem then it will be either a virus or corrupt driver. Depending on what version of windows you have you can run regedit or msconfig from the start/run window and then look at what starts up.
You should also try running antivirus or antispyware programs. Spyware tend to be dumber versions of viruses. Those will keep downloading more and more until the computer is running so slow it may take an hour or so just to put. There are several online sites that will run searches and let you fix them. there are a lot more that do the search and say spend money. A good one is www.microtrend.com
called housecall. It is good at find viruses and spyware and gets rid of them.
Thats covers quite a bit but you should be able to narrow it down and then if it still is slow get specific help to finish it up if it is still slow.