We just use this:
<param name="File" value="${ALLUSERSPROFILE}/Company/Product/Logs/error.log"/>
It works great.
This line can simply be inserted into your current appender configuration:
<appender name="RollingFileAppender" type="log4net.Appender.RollingFileAppender">
<param name="File" value="${ALLUSERSPROFILE}/Company/Product/Logs/error.log"/>
</appender>