|Simple maybe but pointless because there is no rundll.exe file in XP.|
If you mean rundll32.exe then it doesn't go in "c:\windows" it goes in "c:\windows\system32". There is also a little thing called file versions. You have to ensure it is the right one for the operating system and the particular service pack version too.
If you've ever had a service pack you could just copy across the one you already have existing in c:\windows\servicepackfiles\i386 (it's the right version too).
Maybe you were lucky but I wouldn't advise other people to take your advice, especially when there are at least two blunders in your post.
Please come back and let us know the outcome.