My main desktop runs Xandros linux (based on Debian). I'm very happy with it and will continue to use it for my main functions (web development and server management). However, I need to install Windows on it as well for those few programs that don't port over.

The standard advice would be to wipe the Linux installation, install Windows (leaving some space on the disk) and then installing Linux and a bootloader. That's possible for me but I'd rather not deal with re-installing all of my programs and trying to back up all of my data.

Is it possible to install Windows on a Linux computer and keep my MBR intact? If not, can I back up my MBR, then restore it again after installing Windows?

Install windows (Assuming you have free space to partition) and leave linux intact.
Windows will overwrite the MBR.
Then use a linux rescue or live CD and reinstall your boot loader onto the MBR (usually lilo or grub). For lilo this is as easy as chrooting into your linux environment and running "lilo" on a prompt.

Note that you'll probably want to add windows into your boot loader config file before reinstalling the boot loader so you can boot windows too.
Enjoy!

Use a program like QTParted to create sufficient free space on your hard drive (leave this free space you have created unformatted). Windows can't detect Ext2 or 3 partitions (it can see Reiser as unformatted space, for some reason) and check how much space Windows is using to install before you click OK (For example, If your HDD is 40GB make sure Windows isn't installing on 40GB of space!).

For the bootloader, download a SimplyMEPIS live CD as it has a bootloader installer on the OS centre which should automatically detect Xandros and Windows and configure GRUB accordingly.