This is best done on a newly installed O.S.
Open up the Registry by clicking "Start", "Run" and typing Regedit and clicking "OK".
Make sure HKEY_LOCAL_MACHINE is highlighted and click "Edit" on the toolbar then click "Find".
In the box that pops up type
ProgramFilesDir
Regedit will find it, so double-click the word and type in the box the new address you would like.
Above ProgramFilesDir is another word called
CommonFilesDir
double-click this word and type in the same address as you did for ProgramFilesDir, but include "Common Files" at the end.
Continue searching for ProgramFilesDir, there should be at least 4 of them. Change all of them and all of the CommonFilesDir then re-boot and all newly installed programmes will default to that new location.
You will of course still be offered an alternative.
Any programmes you installed prior to this change may need to be re-configured.