If you get a computer that will go into Safe Mode, but will just hang in a regular boot or not load in some way, your OS most likely not the issue.
Once in Safe Mode, try to doing a clean boot. See “How To Do A Clean Boot” for more info. Some Antivirus software can cause this issue. Try uninstalling it just for troubleshooting and re-install once the computer is back up.
If that fails, look for possible Hardware issues. In Safe Mode, check the Event Viewer. I once had 2 out of 5 disks failed on a RAID array that was causing this. Also look at the graphics driver to make sure it matches the video card.
If the computer manufacturer has a built-in Diagnostics, run it. For example, Dell computers can run F12 on start up to run a 5 min diagnostic test.