I'm willing to use Computer Janitor but I'm afraid to lose some important libraries.
Is it secure to use Computer Janitor?
Thanks in advance.
Computer Janitor is being dropped in the next version of Ubuntu - mainly because nobody used it and the serious perceived issues that it could break your system with some of its suggestions.
I would recommend you take a closer look at either Ubuntu Tweak (from here) or Bleachbit (from software centre)
There is an AU question here with regards to Ubuntu Tweak.
I haven't found a similar question for BleachBit - but this AU question may help.