standard errors in summary.lm

Dear R users,

If I have an aov object, how are the standard errors of the estimates in 
summary.lm calculated?

Using treatment contrasts, I would like to use the estimated differences 
in mean values ("intercepts") to calculate the mean values per factor 
level, and for these mean values I??f like to use the model output to 
calculate the standard errors.

Many thanks for your help!
Regards,
Christoph.