This is the situation:

I have a 64-bit machine with Vista 64-bit installed. I want to run something which is being pissy about running in 64-bit mode.

I still have my 32-bit XP disc from my old PC, and so was thinking of dual-booting between the two. I would however like to use my normal Vista desktop while I'm working, and rememeber seeing something whereby you run a Virtual Machine running the other OS, so you have have it in a window within your normal desktop.

Can anyone advise?

A few years ago i used to work with VMware a lot. It was quite good.

http://www.vmware.com/

I've always used VMWare's stuff, they're not the only ones though. VMWare Server is completely free.

http://www.vmware.com/products/server/
http://www.vmware.com/download/server/

The VMWare player is a nice tool too if you want to use other peoples' pre-made virtualised 'applicances'.

Use VirtualBox. It's free, open-source and constantly updated. It works a bit faster too.