Sometimes getting into Safe Mode can be tricky and sometimes it is impossible.
One method that can be used would be to create an ungraceful shutdown.
Boot the computer up as far as it can go and make certain it has settled down (not loading anything else).
Then, unplug the AC plug. Leave it out for a good 10 seconds or so.
Plug the unit back in and power it up.
It will 'usually' go to the Safe Mode screen, telling you that Windows did not shut down properly. It will give you the option to enter Safe Mode in order to fix the problem that caused the shutdown.
This is not something you should do if you feel uncomfortable at all about working on your computer. I would also not advise you to try this more than once.