That's quite well possible.
The specs in our review http://reviews.cnet.com/desktops/dell-inspiron-531/4507-3118_7-32486097.html don't say if it has currently has a SATA or am IDE drive (although, with 500 GB in 2007, I suppose it's SATA), nor if there's any spare SATA or IDE slot available.
My advice: take the case with you to your favorite computer shop and let them have a look inside to see what's possible and what's best. They will be able to install it for you also, for a not too big amount of money.
My Dell Inspiron 531 is running out of space on the hard drive. I'm using the Vista OS. I would like to add a second larger hard drive with Win 7 but keep my old Vista hard drive.
I'd like to make them independent of one another but running them in the same computer. I wouldn't transfer anything to the new hard drive but use it as I do now.
Is this possible to do? What are the complications involved.
I'd appreciate your input.