Task Manager (Ctrl+Shift+Esc): runaway processes, startup apps, performance baselines. Look here first when 'my computer is slow'.
Event Viewer: Windows logs everything. The Application and System logs around the time of the crash usually point at the cause.
Services (services.msc): toggle background processes. Spooler stuck? Restart the Print Spooler service.
User profile lives at C:\Users\<name>. Corruption there causes weird login/permission issues that look like network problems.