When you are in safe mode try to start any programs which will normally run on start up,If any of them crash the computer you have the culprit,after that then try a normal start because just running is enough to fix some errors.
Failing that try james suggestion of restoring the registry,click on run in the start menu and type scanreg/restore be aware depending how often it backs up this may take the registry back a fair way and loose access to recently installed programs.
Usual disclaimer "I take no responsibilty for the smoking lump of plastic which used to be your pc if this advice goes wrong" But it's what I would try if it was mine.