how often depends on what you do. alot of downloads and file deletions, unexpected terminating programs, lock ups, uninstalls, power slams, etc will defrag a system, some worse than others. Close any apps, as stated above, reason is although its SUPPOSED to work it more often than not causes problems when programs are running. Reason being, is that while defrag is trying to organize the clusters on the HDD, if info is altered in any way (read and rewritten) it will be detected that info has changed, and will need to restart. Safe mode is the best place to run a scandisk/defrag.
If you argue with an idiot he will drag you down to his level and beat you with experience.
I am not a fast writer.
I am not a slow writer.
I am a half-fast writer.