Latest version of Defraggler downloads and installs on my Toshiba laptop (xpsp3), however on my wife's HP tower (xp media center sp2 ), it dowloads but refuses to install.

Help!

Hello odysseus99,
Don't know why that would happen but "Stuff Happens".
Be sure your wife has Administrator permissions.Then try again.
Navigate to C:\Program Files\Defraggler and double-click on the uninst Icon if it is there.

Next thing I would try is Dial-a-fix.This is used by many forum members.Fixes everything using MS "fixes".No hunting for the fix.
The first thing to try is check off Box #2 to fix MSI (Windows Installer) and hit the "Go" button.
Dial-a-fix fixed my problems with Windows updates and many other problems.
I use it all the time now.First thing I check when I suspect a problem.The Tools come in handy all the time(Hammer icon at the bottom).
http://wiki.lunarsoft.net/wiki/Dial-a-fix

Good luck,
davey ("Don't leave home without it.") This program is a real "keeper" for 32 Bit WinXP and prior users.