I installed DB2 on windows XP with domain user so it created schema with name of “user.name” and stored all sample database object under that user, now when I connect sample database and trying to select any table it gives insufficient permission error even all object are available in same user schema.

probably because you created the object from within another user
eg
connect to db user db2inst1
create table xx.yy
in that case grant should be given to xx
if create tab auth was give to xx
connect to db user xx
create table xx.yy
this user would have all auth
do some reading in infocenter about this
Authorization

I did not created any object in SAMPLE database, these all objects are created on DB2 installation under mine window system domain name schema.

so : grant is needed for this user/public.....

did you use user.name as schema ??
check if this character . is allowed.....