learn how to use Recovery Mode
Don't do anything until you can restore and have repaired the disk drive.
Post should get moved to either ML (10.8,.4) or iMac forum
Swap out the hdd for an SSD : )
Don't do anything to clean it up unless you don't know how to first remove browser history from within the programs; low on disk space (I consider more than even the 15% minimum, more like 3--40%)
And do you have page outs? more than enough RAM? what actually leads you to ask? Programs aren't opening properly?
Sometimes it is easier to take the long road: just reinstall and restore.
Mac OS X & Mountain Lion Community
General purpose Mac troubleshooting guide:
Mac OS X: Starting up in Safe Mode - http://docs.info.apple.com/article.html?artnum=107393
What is Safe Boot, Safe Mode? (Mac OS X) - http://support.apple.com/kb/HT1564
Creating a temporary user to isolate user-specific problems:
Identifying resource hogs and other tips:
Starting the computer in "safe mode":
To identify potential hardware problems:
General Mac maintenance: