problems with battery´s icon

Hallo, after installing sp1 in my hp compaq 6720s laptop with vista home
premium preinstalled, i
have observed the following problem:
Sometimes when i turn on the laptop or reinitiate it, the battery´s icon in
the notification area of the task bar, appears empty as it would be
discharged, which is not certain because it says 99% available, connected
and without charging. I used to solve the problem reinitiating the computer
because ticking in the battery icon in notification area of properties in
task bar doesn´t solve it.
¿Does anybody knows what is the cause of the problem and how to solve it
without reinitiating?
Thanks in advance
Regards
Clara