Windows: Many types of malware disable booting into safe mode to prevent easy recovery. You can always get around this by reinstalling or restoring your operating system, but that may not be necessary thanks to the free program Safe Mode Fixer.The tool is an executable file that requires no installation. Upon running the executable file and clicking on the Fix button, repairs are applied to your system followed by an immediate reboot. Technology blog ghacks notes:
The developer recommends to disable all antivirus and firewall solutions prior to pressing it. (the fix button) I personally would not recommend that, and would use it as a last resort if pressing the button without doing so did not fix the broken Safe Mode.
Safe Mode Fixer is a free download for Windows 7, Windows Vista and Windows XP.
Windows Safe Mode Fixer: Repair Safe Mode [ghacks.net]