This promotion is discovered by me, so dear webmasters please link to this source on your posts.
1.) Go to the special promotional page to get Free 90 day McAfee Personal Firewall Plus license

2.) click on Add to Cart button below the 90-day subscription (Free Trial) (Screen shot below)

3.) Now login or create a free Mcafee account by filling form and click on I Agree button

4. )Now click 'Download Now' button (exsisting mcafee users) or 'My Account'(new users) link on print receipt page(My account page for new users). Don't close this webpage.

5.) Now check your email inbox, that you have used for the creation Mcafee account. You should see an email from Mcafee (subscriptions@mcafee.com) which contains your account activation link. Click on the activation link which mentioned on this email.(The 5th, 6th and 7 th step is only for new accounts)

6.) Then Go back to the web browser where you left it open (in step 4.)

7.) Click on Download button to download 3 months Mcafee Personal Firewall Plus(screen shot below)

8.) Click on ' I Agree' button .

9.) click on download button and download it. Install it, it does not requires any serial key for 90 days trial(This trick is firstly discovered by me. Enjoy it)