TuneXP
Excellent
Full user review
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"Add to your Tweak COLLECTION, but you'll need others."
Pros
A few tweaks are unique to this little tool, but some are useless (e.g. UDMA-66) for newer machines (i.e. with SATA drives).
Cons
Hardly the most comprehensive tool, but keep it in your toolbox... for legacy machines/purposes. Try the following 3 (the first one is a real powerhouse): NirSoft Utilities, TweakNow, PowerToys/TweakUI for XP.
Summary
You'll want to get the following: NirSoft Utilities, TweakNow, PowerToys/TweakUI for XP.