If your PC is too old for 11.10, you should consider using a light weight distribution instead of downgrading to an older Ubuntu version. Try out lubuntu or xubuntu!
If you still want to your run newer programs on an older version of Ubuntu, this is possible but not relly the suggested way. A new version of Ubuntu is released every 6 months and that moment all programs are updated to an up to date version (not only the core system - but actually it's difficult to say what exactly is part of Ubuntu's core system and what are only extra programs). This enables developers to test if everything is working fine with the distributed programs for every Ubuntu version. Otherwise nobody could ensure that all the different versions of programs are compatible. Most GNU/Linux distributions do it that way. Only some provide also a not so stable branch with really up to date software.
So Ubuntu updates are primarily for security patches and bugs but not for new features. If you still want newer versions of some programs on and older version of Ubuntu, there are several ways to do so. Best would be to find backports. If there are no official backports you can add repositories. The worst would be to find a deb file or even a tar ball, but then you won't even get security updates.