Hard Drive Swapping... 2 computers in 1?

So I have a 3.5yr old HP laptop that actually still runs really well. Like most computers, it gets bogged down with programs etc etc. I use it for 2 purposes: Engineering stuff (some CAD, some other stuff) and recording music. I'd love to back it up, wipe it clean, and reload everything, however alot of the programs I have on there arent easily replaced and alot of it I got for free in College.

My question here is this:
Could I buy a.. say.. 250GB Notebook drive, load it up with Windows and all my music stuff and then swap it our for the 'old' drive. This would in effect give me 2 different computers (kind of like partitioning it, but 2 separate drives). Are there issues with this or is it pretty much plug-and-play?

I'd kind of prefer to keep my CAD stuff completely separate from my music stuff (I might get music from... well... less than ideal sources). I'd hate to loose stuff like that to a virus or anything of the sort.