An issue I have is trying to test different software packages which can be like octopus's in my operating system (like .net development or WAMP). I used to have them all installed and inevitably my computer would run slower and slower.
Now I use virtualBox to create a virtual machine in which I only install the software relevant to the development environmment. It means the actual developmet environment runs faster and it doesnt handicap my working PC.
I have put some instructions on my wiki at http://www.virtualmv.com/wiki/index.php?title=VirtualBox
If you want to make modifications to the wiki entry you will need to create a userid and login. Apologies for this as I know this is not a "true" wiki thingy.
Another big advantage is that I can run virtualBox on another computer (e.g. my Vista one) and can use the virtualBox image without change and it includes all the set up stuff as well - which is brilliant. I found this particularly useful when my work PC was upgraded. My virtual machines were up and running with all their weird configurations once I'd installed virtalBox, copied over the virtualMachines and connected them up.