The Sysinternals Suite is a very powerful set of free tools released by Microsoft. I don't think it needs much introduction. By effectively using the tools in the Windows Sysinternals Suite, you will be better equipped to handle various issues with Windows without spending a penny.
The Suite is a bundling of the following selected Sysinternals Utilities:
- AccessChk
- AccessEnum
- AdExplorer
- AdRestore
- Autologon
- Autoruns
- BgInfo
- CacheSet
- ClockRes
- Contig
- Ctrl2Cap
- DebugView
- DiskExt
- Diskmon
- DiskView
- DU
- EFSDump
- Filemon
- Handle
- Hex2dec
- Junction
- LdmDump
- ListDlls
- LiveKd
- LoadOrder
- LogonSessions
- NewSid
- NtfsInfo
- PageDefrag
- PendMoves
- Portmon
- ProcessExplorer
- ProcessMonitor
- ProcFeatures
- PsExec
- PsFile
- PsGetSid
- PsInfo
- PsKill
- PsList
- PsLoggedOn
- PsLogList
- PsPasswd
- PsService
- PsShutdown
- PsSuspend
- RegDelNull
- RegJump
- RegMon
- RootkitRevealer
- SDelete
- ShareEnum
- SigCheck
- Streams
- Strings
- Sync
- TcpView
- VolumeId
- WhoIs
- WinObj
- ZoomIt