If an users update running old version of the antivirus, Windows 10 will automatically disable that and use the inbuilt Windows Defender as the security software.
ESET team advised to upgrade their antivirus software before taking place windows 10 update. It will solve the issue and users can upgrade or update without any hassle.
For more detail about Windows 10 & ESET Products known issues visit this knowledge-base of ESET
http://support.eset.com/kb3733/
If you are running windows 7 or windows 8 with ESET antivirus installed and want to upgrade to the Windows 10 then please read and follow the following official instructions by ESET
http://support.eset.com/kb3747/
If you have already upgraded to windows 10 and ESET get disappear or causing some other error. First step is to update the ESET to the latest version. Most of the issues will be resolved by updating to the latest version. But if still you are facing issues then please contact ESET customer care or refer to the above referred KBs.