Ultimate Windows Tweaker
Windows users should consider checking out this very useful utility; Ultimate Windows Tweaker. With over 150 options to change, it is indeed ultimate when it comes to tweaking Windows.
The tool works well for Vista and the latest Windows 7 RC1 for both 32 and 64 bit versions. Prior versions of Windows are not supported. No doubt RC1 have introduced some new features such as dragging windows to the edge to make it fill up half the screen (useful for widescreen monitors) and shaking an open window to return to desktop.
For some people who are annoyed by this, Ultimate Windows Tweaker allows you to turn it off. Customization options are split into 7 distinct categories, which are Personalization, UAC, System & Performance, Security, Internet Explorer, Network Optimization and Additional Options.
The list of tweaks cover stuff like turning on check boxes to select files, restore folders at startup, show drive letters (or hide them), disable balloon tips, enable/disable UAC, force DLLs to unload from memory when not in use, turning off autoexec.bat file (viruses love this) and a host of other options too lengthy to list out here.
Most, if not all, of the changes can be achieved via editing keys in the registry, but Ultimate Windows Tweaker collects all these little tricks into a cohesive, easy to execute program which is also portable and can be carried around in a pendrive where-ever you go.
XdN Tweaker
If you are looking for alternatives, consider checking out XdN Tweaker, which is not as fully feature rich as UWT, but does the job pretty well too.
Happy tweaking Windows!