Activities with negative float

Hi...my problem is the follow...i have a group of activities that show a negative float but by the times the float must be positive...by example:

Project Start 04OCT2009, Project Finish 21OCT2009
Activity 1 ES=04OCT2009, EF=06OCT2009, Float= -24.

I think that the problem is in the relationship but all the activities have FS relation whit no lags...have the same calendar, have assigned resources and why that negative float is showing but Primavera?..

THANKS FOR ALL THE HELP.