Do you have a legitimate store-bought copy? Did you follow all the instructions while installing it? Was it a "clean" install or an "upgrade install?" What exactly is the nature of "software shuts down?" i.e. when does it shut down, at what step in the boot process, or is it after the boot process, what exactly are you running or doing when it shuts down? Does it shut down only when you launch certain software, on launching any software? Does it give you any error messages before or during the shutdown, or for that matter, during the boot process? Have you run both antivirus and anti spyware removers, and did they report any problems? Did you do this before installation? Did Vista ever work on your system, or did it begin shutting down immediately upon install? If it worked for a time, then began shutting down without warning, that suggests something changed or was installed between when you installed it and when the problem occurred. Is all your hardware up to date on its drivers? Did you choose the 32bit or 64bit installation, and does that match your processor?
We use both winXP and Vista (32bit and 64bit) at home and we have not encountered any issues with either installation.