Here's a web page full of mostly registry-changing tools for Windows XP and Vista. I have recently used some of them, and I have found all of them to be safe. Note that for most of the items, there is a "undo" file to download, in case you don't like the changes made by the particular registry patch. So far I have only read a little over 200 of them.
Some of the more interesting tools you will find are some tweaks for making XP work like 98 and one to increase the number of simoutaneous downloads that Internet Explorer can handle, from three to ten.
I have made some of these changes without using a patch or utility, but for those folks that don't like messing around in the Windows registry, these may be of use.
NOTE: If your anti-virus software warns you of a "malicious" script, this is normal if you have "Script Safe" or similar technology enabled. These scripts
are not malicious, but they do make changes to the System Registry.
Click this link to see a variety of Windows XP and Vista tools.