About three years ago I acquired a ThinkPad 600E. It was supposed to replace my old 380D, but bopeepsheep appropriated it. Toward the end of 2004 the hard disk died and she upgraded it at considerable expense to an 80GB one. So this machine has a couple of years' worth of data on it. Then, while we were in Texas, the screen got cracked. I found a ThinkPad 600X going cheap on Ebay, and the hard disk was transferred into it with no problem (save a momentary loss of resolution while Windows loaded new graphics drivers in the background). So this is the "old" laptop.
We then shipped the cracked laptop off to our insurers (or, to be more precise, the firm that deals with computer-related claims on their behalf). To my complete surprise, they responded by supplying us with a brand new ThinkPad R51e. Bargain! This is, obviously enough, the new laptop.
So. Given a new laptop with a blank (apart from the operating system) 40GB disk and an old laptop with a populated 80GB disk, the plan was to switch the disks. I tried this toward the beginning of June, and to my complete lack of surprise it failed to boot (but why? Linux wouldn't have had a problem with that). My solution to this is to boot the Windows CD and press "R" to "repair" the installation. There wasn't time to do this, so I swapped the disks back and put the Windows CD away with the rest of the new laptop's documentation. bopeepsheep offered "A good time to do this would be while I'm away."
 Apparently there's a way to make it work without going through all that, but I didn't know this.
In mid-June, bopeepsheep and smallclanger went to the seaside, but I stayed at home because I didn't have time off work. I began by attempting to copy as much data as possible from the old laptop into a backup folder on the new laptop (because that's the machine that had the most free space on it). This wasn't easy but in the end after two nights I think I had almost all the contents of the disk other than the Windows system directory and the two largest folders: "My Music" and "My Videos". (It's hard to be sure because I was using Knoppix and it got very arsey with the network card and crashed before I could verify everything, and by that time it was too late at night to set it all up again.)
So then I switched the disks and went to get the Windows CD, only to discover that someone had swiped the CD, leaving just the sleeve. I searched, but didn't find it. And so I had to switch the disks back again. The next day I went to fetch bopeepsheep and smallclanger back from the railway station. And the day after that I got a colleague at work to burn me a copy of the missing CD. But I then didn't touch the machines until last week, when bopeepsheep and smallclanger went to visit land_girl.
Now I should have asked whether it was still the plan to switch disks, yes. I didn't. My plan was to have both machines up and running, so that to a large extent it didn't matter which was which. It didn't quite work out like that.
In fact, I had the old disk (the one with all the data on it) in the new machine working perfectly by the time bopeepsheep returned. Or so I thought. There was just one problem: if I had thought to test Microsoft Office I would have discovered that it was sulking about being asked to run on a different hardware configuration from the one on which it was licensed. (There are a couple of ways in which this could have been fixed.)
The difficulty lay with the new disk in the old machine. It was running XP Home, not XP Professional, so my CD refused to repair it. This wasn't that much of a surprise to me. What did surprise me was that it didn't offer to upgrade either; it would only install a fresh system on top, with probable loss of all accounts and settings and possibly also the contents of "My Documents". I wasn't prepared to do this, so I left it alone.
But two weeks on from my previous attempt to switch the disks, the new disk was no longer a disk with no data on it. I tried to redeem the situation slightly by booting Knoppix and setting up a share so that all the data could be accessed by the other computer. This took much too long to get right and bopeepsheep had already gone to bed by the time it was working. And by the next morning it had crashed. (I haven't used many Linux distros, but Knoppix seems to be the least stable one I've tried. I was using 5.0.1 and it wouldn't even boot without "nodma"; I also have a disc of 3.8 and that oopses at boot time no matter what parameters I give it.)
And so the disks have now been switched back. The "repair" of the old disk now back in the old machine didn't go entirely smoothly, so it took rather longer than intended. But both machines were working again by Sunday evening.
 (It came up with the dreaded The file 'asms' on Windows XP Professional CD-ROM is needed message. Microsoft KB says this is because your system has lost contact with the CD drive, and that proved to be correct. I applied the fix described, only to have the keyboard and mouse stop working, which made it impossible to continue with the install once it put up a box on which I had to click "Next". After a very long sequence of rebooting, messing with the registry and restarting the install, I discovered that the correct fix is to leave the registry alone and just restart a couple of times — it will then magically discover the CD drive and continue as normal.)