Why does my computer keep giving me a warning about a virus , but then when i run a scan with windows defender it says there are no viruses?

The best thing you can do is download and install Linux, Ubuntu.

I quote the following from the Windows Defender wite at:

http://funadvice.com/r/bnnnc88vi “… Important note about fake antivirus

Reports of rogue antivirus programs have become more prevalent lately. These are programs that generate misleading alerts and false detections of malcode in order to convince users to purchase the illegitimate security software. For example, programs such as Trojan:Win32/Antivirusxp and Program:Win32/FakeRednefed may display product names or logos in an apparent unlawful attempt to impersonate Microsoft products. These products may represent themselves as “Antivirus XP”, “AntivirusXP 2008”, “WinDefender 2008”, “XP Antivirus”, etc.

If you believe your machine is affected by malicious software or potentially unwanted software, you can use Microsoft Security Essentials or another up-to-date scanning and removal tool to detect and remove malware or other potentially unwanted software from your computer. For more information on Microsoft security products, please visit our security products and services website. Additionally, you can get virus-related assistance at no additional charge from Microsoft in the United States and Canada via a toll-free support hot line, (866) PC-SAFETY (727-2338). For support outside the United States and Canada, please contact Microsoft Help and Support worldwide. …”

– Best wishes - Majikthise.

agreed with above statement. the virus/ malware etc pretends to your computer and give you a safe warning when in fact its asking you to let the virus in. dont listen to any of these thigns just trust your real internet secuirty.