Smartest thing I've read all day. It is literally a perfect match to the original question, which is probably the dumbest thing I've read all day (drive image your work laptop, smoke it and install your own warez, and restore the drive image before giving it back to them.)
OP - here's the one piece where your plan fails : the active directory connection establishing your machine as a trusted member of the domain, and your user as the domain with the same name ... disconnects if it hasn't been refreshed in a while. I don't know how long it takes, but it happens. And it is a particularly uncomfortable discussion with corporate IT explaining why, given that your machine looks exactly like it did when they gave it to you, and you have been using it for a few months. The question is going to come up 'What did you do to it?' and you are going to answer just like they expect you to 'Nothing.' ... and it goes downhill from there.
Technical answer for you is same as Anrego : USB Thumbdrive install of Linux : Pen Drive Linux has a zillion distros you can pick from, and they give you step by step instructions on making it work.
If technical answer #1 doesn't work for you, here's technical answer #2 for you : remove the work hard drive, install a new hard drive, install your own OS on that and swap out drives for work / pleasure. Downside is limited to the danger of physically borking the work drive while removing it or storing it while it is out of the machine. Explaining how you managed to mangle the SATA connector on a work laptop is a very difficult discussion.
Personal preference answer is also same as Anrego : don't do anything on your work laptop that you wouldn't do with representatives from corporate HR, IT, your boss and his boss standing over your shoulder. Buy a cheap used netbook for $150 on Craigslist and take it with you to do your warez/internet surfing/pr0n viewing.