I have windows defender. I also have Mcafee. Do I need to have two security systems?

1) It depends which McAfee you have. McAfee is an established antivirus provider, that also provides security suites including firewall, antivirus and antispyware.
2) While you should only run one antivirus program, you can safely run more than one antispyware program, and Windows Defender can supplement whatever protection you have from McAfee.

You can run both if you want. Windows defender is not a true anti-virus as we are used to. McAfee is not that highly rated. It is really huge in the image files and will slow down your operations. Do yourself a favor and get Kaspersky, Advent or AVG. All are good and are small images.

i wouldn't recommend having two also i wouldn't recommend having either of those two you mentioned i would go to www.download.com and get avg antivirus it's free

windows defender is an anti spyware, mcafee is an antivirus. you can have those two running without conflict.

No No No!!, don't ever use 2 security softwares if you're not in the emergency situation. 2 programs are used at the same time means your computer'll be slow down because your RAM cannot supply enough space for 2 security softwares.(Sometimes 2 of these programs or others will have problem with each other, I can't explain it clearly but it looks like they're "fighting each other" (Mcafee will be limited by Windows Defender and Windows Dfender will be limited by Mcfee too so your PC cannot work properly) .That's my experienced .Just you the best security programs and always update it.
Good luck!

Windows Defender is an anti-spyware only. It is not a "security system" or suite. It provides a nice extra layer of protection to your McAfee. (I assume you have the suite)

The combination of the 2 should provide you with good protection.