Care to share which SL version you have? The following is just some basic troubleshooting until you provide us w/your current SL version. Please detail what you have done so far in the way of troubleshooting? Need this info to avoid the "been there done that" scenarios.
If you found no solutions to your problem(s) listed in the various Knowledge Base Articles and those listed in the More Like This threads over in the right column, see if the following suggestions are of any help:
Except for the keyboard & mouse, disconnect all devices/peripherals from your computer.
Step 1: Restart into Safe Mode by holding down the Shift key during startup. The comp will run a complete & lengthy file system check, then start up w/only the required kernel extensions & w/o running any Startup items that you installed. If you are running OS 10.4 it will also perform some font finagling. Try starting up normally again. If you are still stalling, restart in Safe Mode & move on to Step 2........... Mac OS X: Starting up in Safe Mode
Step 2: Open Library>Startup items folder-the *root* level Library, not the one in your user name folder.
Look for potentially problematic 3rd party items such as virus scanning software components, etc. Remove 1 or more items to a location outside the Library folder, such as the Desktop, then restart normally & see if your Mac stalls. It it does, try removing all the items in the Startup items folder & restart. If all’s now well, place the items back into the Startup items folder 1 by 1, restarting each time you put one back. If and/or when the stall resurfaces, you just identified the faulty component.
If the above does not work, then it’s time to move on to Step 3.................
Step 3: WARNING-This step can be tedious-removing .plist files from the /Library/Preferences folder.
Remove the entire Preferences folder from the Library folder on your start up drive then place the files back 1 by 1 until you see the problem return. *The Mac will automatically create a new Preferences folder in the Library folder*Alternatively, you can take half of the items out (all non-Apple .plist file, for example), restart to see if your Mac stalls, then move to a smaller set of items, repeating the process until the culprit is found.
Or you can use Preferential Treatment which is a *FREE* utility that does all the work for you.
Hang in there & good luck!!!!