I'm trying to use a program that requires the d3d8.dll in the system32folder it is 1MB however the d3d8.dll that it used for my program is 3mb when i try to add the new d3d8.dll it is almost immediately overwritten with the 1MB version again. Could I stop this from happening? If so would it harm my computer?

Try it from CMD and if no luck, do it from real DOS. Computer Components

"...it is almost immediately overwritten with the 1MB version again." Windows File Protection?

The most likely reason is that there is a backup copy of the 1mb file in the \System32\Dllcache folder and to avoid the immediate reinstallation of the original you must delete it [or move it to a safer place to see what happens and don't delete it until after your next reboot...just in case] from the aforementioned folder. Then overwrite the 1mb with the 3mb in the \System32 folder and wait a few moments to see what happens. Then if you get a message box requesting [demanding that is] that you put your XP cd in blah blah you can choose to keep the new file and tell SFC where to go. You can put a copy of the 3mb in that folder as well if you want.