What to avoid cleaning

Some things that you should not clean by default or never clean. This comes from my PC Cleanup article.

• The Memory Dumps cleaning option will remove any memory dumps that are generated when a program crashes on your computer. Many technicians can use these memory dumps to tell you exactly what caused the crash. These memory dumps are also sent to a crash analysis center at Microsoft when you use the Error Reporting feature. Reporting these errors and sending the crash dumps help all users because hotfixes are often issued to fix the very problem that made the program crash.
  
• The Windows Log Files cleaning option will also remove another memory dump file called user.dmp that is generally placed in %ALLUSERSPROFILE%\Application Data\Microsoft\Dr Watson\. This is also useful for technicians to help diagnose errors that cause programs to crash, similar to the Memory Dumps above. In addition, this cleaning option will also remove log files from both program installations and Windows Updates. A user or technician can refer to these generated logs to determine the source of the installation problem if a problem occurs.
  
• The Old Prefetch Data cleaning option will decrease your system performance. The impact that cleaning/deleting the prefetch data has on your system is well documented by technicians and Microsoft staff. Prefetch is designed to allow your applications to load faster.
  
• The Hotfix Uninstallers cleaning option will remove every hotfix uninstaller. This is a bad option for CCleaner to have. If you have this option checked and you remove your hotfix uninstallers, should you ever have an issue with an update (hotfix) from Microsoft, you would not be able to uninstall the problematic hotfix. This could potentially result in your system being crippled.
  

In fact, the Hotfix Uninstaller cleaner caused a lot of problems relating to IE7, SP3's preview downloads (the betas and RCs), and other Microsoft preview programs.

These things getting cleaned can be a real pain for technicians. I'd recommend that there not even be options to clean these kind of things at all.