To an extent it depends on what you've "messed up" and whether you made a recovery disk (does anyone ever?).
I must admit to making a basic assumption that Microsoft haven't changed the basic procedure for XP from NT/2k whereby you can enter some basic repair options from the set up menu by booting from the windows CD, however I've had mixed fortunes using this, sometimes it just fails miserably (mainly on NT4 which has much more limited options) sometimes it partially recovers windows but not your application settings and so on, but as a last ditch effort before reinstalling from scratch its worth looking at.
On the off chance you've made a recovery disk and kept it up to date (ie rerunning rdisk everytime you install or remove any software or change settings) you may be able to to restore your installation back to what it was by booting from the recovery disk.
The only thing I have noticed is that since installing Mandrake I've been completely unable to get windows to take back the MBR, I boot via Lilo now whether I want to or not