OK - this was fun. Nearly made a complete pig's ear...
Situation: Friend's laptop had a dual-boot config, with Windows XP and SUSE linux 10. She wanted to get rid of the Linux, to have all the hard drive for XP.
Here's the idealised version of what I did - ie, here's what to do without my mistakes:
Make sure you have an XP install CD, so that I can use the recovery console [an alternative is a Linux LiveCD]
Make sure you can log into the Recovery console for the c:\windows installation [In this case, I couldn't and had to go back into Windows and reset the hidden administrator password, using the XP Pro Admin tools - you can do this in Safe Mode with XP Home]
This isn't necessary if you're going to use a Linux LiveCD, but before doing anything, run the CD to check you can find the tools you want.
Remove Linux Partitions: In windows, right click "My Computer" icon, select "Manage", choose Storage -> Disk Management, and delete the two extra partitions that aren't "C:", (good pictures here)
Mark the C:Windows partition as Active. In the disk management console, right click on C: and "Mark Partition as Active" (my crucial mistake - didn't do this, couldn't get the system to boot, used a Linux LiveCD, Xubuntu, partition manager, to mark the partition as 'bootable' - and then it worked)
Restart with the Windows CD in, and go into the Recovery Console, type 'fixmbr', then 'exit', and reboot without the CD.
As I've mentioned, you can use a Linux Live CD if you haven't got the Windows install CD, You can use "fdisk /mbr" from a linux command prompt, instead of "fixmbr" - google it