Startup files are rarely the cause and in any case they only account for a fraction of what could be causing it.
More often than not it's caused by a bad third-party device driver, or software that installs a virtual driver such as Antivirus software, third-party firewall, or CD burning software. Those are where you should be looking for the most likely cause. Try to remember if you've recently installed anything like that just before the problem started.
If you remember a likely candidate, uninstall it in safe mode, then restart.