+1 to what
konradh was saying. I have had similar experiences. Although a complete rebuild was not required in may case I wasted days just pulling a trojan out.
If you are running a serious income dependant studio a sep machine is the only answer. But even so you can still get into trouble and in my case the trojan mentioned above was due to a client bringing in an infected USB stick or USB drive. You can be off the net and still get into trouble that way and very easily might I add.
I have MS Essentials running all the time on my office machine and now before I just plug client USB sticks or drives in I do a scan first on another machine. You would be amazed how often MS Essentials has found and trapped something.