Changing Default Install Path

I find it folly to install my applications on the C drive, especially the larger applications. Doing so makes the C drive so large that it is cumbersome to make the image backups that I think are necessary to insure an easy recovery from any problem causing a failure to boot. In WinXP, there is an easy way to change the default install location by changing ProgramFilesDir at HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindows Current Version. I changed that to a specific folder on my D drive and have used that for the last 8 years with no problem. It appears this registry key is only used by install programs to let them know where you want your programs installed.I really miss this wonderful feature in windows 7. Actually windows 7 has the same registry key, but unfortunately it does not seem to be used for the same purpose. I'm not sure exactly what it does, but at least for some programs, Windows seems to use it as a path to find the program when it is to be executed. Of course that would make little sense because there would be no way to actually change the key without breaking something. (What sense is there in having a registry key that can't be changed?). I have tried changing this key on two different systems and eventually it did lead to problems on both systems. So now I'm back to laboriously changing the path from C:program files or C:program files (x86) every single time I install an application (which it turns out I do quite often).

Is it possible that a script could be developed to search for C:program files or program files(x86) in an edit box and replace it with my preferred path with a single hot key? That would at least take most of the drudgery out of it, although it would still require choosing the "custom" install on many applications, not to mention the occasional errant applications that always install to the default install path with no chance for user intervention.