Might just be timing. Try F8 just after the Dell splash screen goes off.

Like in comedy, timing is everything. After the splash screen you will hear a beep. Immediately after the beep, press the F8 key and you ought to see a series of options, one of which is SAFE MODE.

By chance you using TweakUI? If you are look under the "Boot" tab to see if the functions keys are disabled. This will prevent the F5 and F8 keys from being used when the computer is booted.