In your boot menu,what is showing first either Windows Boot manager or Ubuntu Boot Manager?
uninstalling an operating system isn't the same as uninstalling software. If you need the space format that partition and expand windows if that us what you want to do(give windows more space) but you need to make sure you reinstall windows' boot mgr on the mbr partition.
WARNING :Any damage migh be happened. So before you do anything on your computer.
Be sure backup important data before starting.
It should be fine if youR boot manageR was from windows xp.
Just log into windows xp and start disk management and delete the partition which was used by Ubuntu.
Finally, turns off all system attribute from the boot.ini file on the c:\ (type attrib -r -s -h c:\boot.ini).
Delete the entry which was point to Ubuntu.