Thank you to all who responded so quickly ! 
 And special thanks to Jim Harmon, that put the subject in a very nice form.
 I will list his reply. 
 SUMMARY:
 Your problem is seriously compounded by the use of a single partition to
house root, /var, and /opt.
/var and /opt house all your applications.
Many applications have logs.
All the logs will grow continuously, unless you add them to the script
that clears them in cron.
check /var/adm and /var/log for the primary logs.
You'll probably find /var/adm/messages is huge, and that /var/log/syslog
is also huge.
You'll want to "trim" the following files as well:
	/var/adm/utmp
	/var/adm/utmpx
	/var/adm/wtmp
	/var/adm/wtmpx
and anything in /var/adm/acct/
		/var/adm/log/
The simplest method of "triming" these files is to MOVE them, TOUCH
them, CHMOD, CHGRP and CHOWN them, then RM the old file in that exact
order.
Example:  mv /var/adm/wtmpx /var/adm/wtmpx.old
	  touch /var/adm/wtmpx
	  chmod 644 /var/adm/wtmpx
	  chown adm /var/adm/wtmpx
	  chgrp adm /var/adm/wtmpx
	  rm  /var/adm/wtmpx.old
Why move it first?  You MIGHT have to restore it quickly.  It's an
important system file that must be available all the time, or the sytem
can hang, and won't boot, because it can't log the shutdown from the
console.
IF you're using the Accounting software, it will grow infinitely large
logs also, and they should be trimmed periodically.
My strongest recommendation is to NEVER keep GROWABLE filesystems in
ROOT.  You should ALWAYS have seperate partitions for /var and /opt.
Behind, I have deleted the following files:
/var/adm/messages.* 
/var/cron/log
/var/log/syslog.*
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			      Stella Maris Resende
	   Laboratorio de Quimica Computacional e Modelagem Molecular
			 Departamento de Quimica - ICEx
		      Universidade Federal de Minas Gerais
			    Av. Antonio Carlos, 6627
		31270-901, Belo Horizonte, Minas Gerais, Brasil
			     FAX: (55) 31-499-5700
			    PHONE: (55) 31-499-5776
		       e-mail: stella@netuno.qui.ufmg.br
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