I think I have a virus but Windows Defender has not found anything.
Should I run the scan in safe mode? Would that make a difference?
thanks.
Yes, you could optionally run Windows Defender in Safe Mode with different results.
Alternatively, you could get a second opinion from several trustworthy free third-party on-demand malware scanners. Windows Defender does not detect what others may consider malware. Since you don't give details of why you think you have a virus try those
listed below.
Malwarebytes Antimalware-Free Edition (uncheck any offer of Premium trial during installation)
http://www.malwarebytes.org/products/malwarebytes_free/
How to use Malwarebytes' Anti-Malware to scan and remove malware from your computer
http://www.bleepingcomputer.com/virus-removal/how-to-use-malwarebytes-anti-malware-tutorial
AdwCleaner
http://www.bleepingcomputer.com/download/adwcleaner/
TDSSKiller
http://support.kaspersky.com/viruses/disinfection/5350
ESET Online Scanner
http://www.eset.com/us/online-scanner/
~bhringer
.