The easiest way to replace uxtheme.dll, or any other dll, is to delete the backups of the dll(s) you wish to replace in the dllcache folder (in the system32 folder).
For XPsp1(pro or home), you have to check another backup folder (don't worry, it can't be in both folders, only one or the other) which is X:\WINDOWS\ServicePackFiles\i386 (X being your system drive)
Once you've deleted the backups, simply right-click-move the dll from the system32 folder to your desktop, click "cancel" in the popup that asks for your Install disk, and "yes" in the popup after that. Then right-click-move your hacked (or whatever) version of the dll to the system32 folder, again, click cancel in the first popup, and yes in the second one.
Restart, and either archive the original (for restoration purposes) or simply delete it. This worked for me when I replaced a hacked version of uxtheme.dll to run a Mac OSX Jaguar Theme (very nice) to give my windows a MacOSX look.
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