Why can’t i install programs on VISTA in a two partition environment?

I have XP Pro running succesfully on my first primary partition. On my second partition I have just installed Vista Ultimate.

But when I try to install software, (the same software as I have on the XP partition) Vista gives me an error – saying the program has stopped working. I’ve tried it so far with my ISP software, my soundcard drivers/software etc. Surely I can install this software on both partitions??

Hope you may help!

Jean-Paul