Some files may be stubborn (especially if they are dodgy files) & may not let you delete them in the normal, simple way. For instance, you may get an error telling you that the file's being used by another program & so it can't be deleted.

Booting into safe mode is easy...just keep tapping the F8 key as you start your computer.
Safe mode

The other 2 apps I mentioned (Unlocker & DelinvFile) are easy to use & very handy for getting rid of files which otherwise won't delete. Of the 2, Unlocker is probably the easiest to use. Once it's installed, right clicking on a file will give you the option of letting Unlocker deal with it...it will tell you whether a locking handle is found & if not, you can still choose to delete using Unlocker. If files can't be deleted then & there, it will give you the option to delete the file when you next restart your computer. It's really user friendly.