Which version of windows. If it's XP you can replace the old MBR using the fixmbr windows program from the windows rescue disk, or the installer disc in management mode. You can remove the Linux partitions in the Windows disk manager, or from your Ubuntu luve distro or the install disk. Some Linux distro's save a cooy of the original MBR in the /boot/ directory. You might want to copy it to a usb flash drive. Using the dd program from Linux, you can restore the first 446 bytes of this file to the MBR to restore the original. The last 64 bytes cintain the original partition table, but you may have reduced the Windows partition to leave room for Linux.
If you used wubi to install Ubuntu, then Linux is in a file inside windows. In this case, you only need to restore the MBR and delette this file. Also check if Wubi has an uninstall option. It may do both. Note: I've not used Wubi, so I don't know if this is the case.
If you use Vista or Windows 7, the drive management programwill allow you to resize partitions. After you resize a partition, windows may want to run a chkdisk program the next time you reboot.