What do I do so that I could launch firefox on windows 7 if it is installed on windows xp. I tried going to the drive where it is installed. And launch firefox from there but the firefox that is installed on windows 7 keeps on launching. I even changed the firefox.exe into something different and cleaned the registry using tune up utilites but still don't work. what do I do.
Don't think that Windows XP and Windows 7 can play along like this, imagine that your computer has two different operating systems (which it does). You need to install a copy on Windows XP and a copy on Windows 7.
Just because you share some hard drives between the two doesn't mean they will place nice.
If you really want to save disk space, you could consider installing Firefox to X:\Firefox (where X is a common drive) but unless you keep the configurations in sync (eg if you install an extension it may be installed to X:\Firefox but your user preferences for the extension may be stored in %AppData%\Firefox which is different between your two operating systems) bad things will happen. I do not recommend this approach.