what can i clean and what cant i clean with out getting errors windows 7
Don't clean your Registry - unless you know what you are doing.
IIS log files seems to cause a Task Scheduler problem. I don't know if this is fixed.
Don't wipe free space - it's useless.
And don't clean the old Prefetch files - it's useless too.
Read and decide for yourself: http://docs.piriform.com/ccleaner