opening old apps in Win7 64bit

I don't pretend to understand exactly why some of my older programs won't run on 64, but what I want to know is whether or not there is some kind of "simulator" or software environment where I can get these old softwares to run? Some are old Windows and some are DOS.

Thanks.

Depending on the edition of Win7 you have (eg: Ultimate), you can download "XP Mode" from Microsoft, which is a virtual machine with XP installed on it - and it can be integrated with the normal Start menu etc.