celaning the xer file is only 1/2 of the solution you have to clean your database also that is creating the large files.
here is a link to a free cleaner
http://scheduleanalyzer.com/PurgePOBS.exe
Now make sure that you clean EVRY xer that you get from ANYONE.
next you will have to clean your existing database here is how
Go to Start Programs Microsoft SQL server 2005, SQL Management Express Studio
the sever name is the name of the computer
Authentication is SQL Server Authentication
User name is sa
password is Prima123Vera then click connect
Expand the databases highlight pmdb$primavera
Up top select new query
then type delete POBS; then right click and select execute
now close sql
done