So my questions are:
- Should I add the external modem and CD-Rom writer and get them running under Windows before I re-install Linux?
I ran Xandros for a while, and liked it a lot althought it had the same fault as Debian "stable", which is being a bit behind the rest of the world in software versions (which is why I am using Gentoo now). I never found the boot manager to be particularly fragile with respect to adding and using new hardware, but I didn't dual boot either. Also, if you are fairly confident that the harware works, then you can just start getting it working in Linux right away. But in either case I don't think that it would matter.
- Should I install Linux on the Thinkpad standing alone before trying to get the external monitor, modem, and CD-Rom writer working under Linux? Or should I try to install with the Thinkpad in the port replicator?
That is probably the way I would go, as using any Linux for the first time on a laptop is a bit of an adventure. Check out
www.linux-on-laptops.com for more information on your model of laptop with respect to "Debian", as Xandros hasn't been around for a while and there's probably not many how-tos written for it. You can also check out
www.debianhelp.org for more information in addition to the Xandros forums.
- Is there any way to have Xandros install its LILO somewhere else than the MBR of the Windows partition?
I asked that very same question on the Xandros forums, and Matt (or Tanter, I can't remember which) explained how to move it over. Basically, you can tell the installer not to setup a boot record, and then make a boot floppy. Once you boot off the floppy and get into Xandros, you can then move the boot manager to just the working partition. But, in reality, several people have stated that using LILO is better than using the Windows boot manager anyway. Just search on the Xandros forums for lilo and my name (clutch).
- Is there any way of saving the MBR to diskette and re-installing it if I have another crash?
Yep, during the install as listed above.
- Can anyone comment please on the Acronis software, which has its own 'boot manager' as well as partitioning software? Or is there any other way of double booting rather than using the Xandros LILO?
Don't know anything about that.
Incidentally, should I be asking each question in separate posts rather than all at once?
This is your first time, so it's fine. You will have a massive array of questions so it isn't a big deal for now

.
One *strong* recommendation. If at all possible, make a backup image of your harddrive so you can restore it if you system bombs out. It is *very* likely to happen, although many people in the past have made it through a dual boot option without incident (although a bit frazzled). In general, just back everything up. The Xandros installer and partition manager is pretty nice, although I had issue with suspend/resume where the system would get really sluggish, and eventually I stopped using Xandros for that reason. Others say it works fine for them, but for me it didn't. Also, if it doesn't work on the first try, don't get too upset. It sounds like you don't have a ton of Linux experience and you are doing a fairly complicated install. Just stick with it and it will all work out, eventually.
Here's a link breakdown for you:
For the laptop
http://www.linux-on-laptops.com/For the modem
http://www.linmodems.org/For the OS
http://forums.xandros.comhttp://www.debianhelp.org