Great article at Lifehacker on Windows maintenance tools that anyone wanting to run a healthy PC should have installed.
Apart from CCleaner, Revo Uninstaller, Auslogic Disk Defrag, and Spybot – Search & Destroy, I would also add some in case virus/malware/spyware removal is required. RootKitRevealer, Ad-Aware, ATF-Cleaner, Avenger, HijackThis and Trojan Remover.
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