I had XP & Ubuntu dual-boot on my desktop. I had major snags re-partitioning as Windows would not recognise the Linux partition. I solved it by deleting the ubuntu partition from within Ubuntu - using the Ubuntu live CD.
On my messing about laptop which I experiment on, I had Ubuntu & XP also. During a botched Ubuntu update I wrecked the HD and could not boot to anything. Most (free) formatting software would not deal with none NTFS file systems IE not Windows. Had to remove "superblocks" (dunno what they are but it worked) using Testdisk on Ultimate boot CD.
Sorry can't offer anything more descriptive but I encountered many snags trying to go from Ubuntu dual-boot to just windows.
Ultimate boot cd:
Ultimate Boot CD - Overview