OS boot

I have XP and I want to install Vista in the same HD without overwriting my current OS and be able to switch OS

What software should I use for that?

You don't need any kind of software to do that.

Just install the other OS in another partition (not on the same partition where the XP is on).

Then from the My Computer Properties (in XP) advanced there is startup (& something) menu, from there you can chose how much time it can give you (from the starting your PC) to switch between OS.

There are some programs to do that:

Partition Magic

Or

Acronis Partition

Well.....I don't really know, but I think it won't work that much.

I mean that it might not give you the option that you need.

I think you may be able to do that from the windows but I can't till how right now, because I am not setting on XP on this moment.

After you install them you need to restart your PC, then (as i remember) chose create a partition, then divide the space between them (if it 80GB make one 40GB and the other also 40GB or as you wish).