Here's how it usually goes:
EDIT: just to correct, you don't "uninstall" Ubuntu in any way, like you don't "uninstall" any other operating system I'm aware of. You just "get rid of it" by overwriting the bootloader it used in MBR, and removing it's partitions and either creating new or extending your existing partitions over them. That's not "uninstalling", it's "replacing".
- get a Windows boot disk if you don't have one yet - you might need it if things get bad
- once you have it, try that it actually boots your Windows
- boot Windows from your Windows setup disc to enter the Recovery Console (XP), or if you know it's installed to the harddisk (it should be, if you don't have a setup disc like some don't seem to have - just a recovery partition), boot from bootdisk (if needed) and before Windows actually loads, use F8 (I think it's that) to enter the Boot menu, from which select Recovery Console
- once inside Recovery Console answer any questions asked and run 'fixmbr' which should replace Grub, or whatever bootloader you use, with Windows' own bootloader
- if it says it succeeded, reboot and see - you should not see anything related to Linux, just the Windows starting to load after BIOS gets out of the way. If this is not the case (you get strange, frightening errors, still some other bootloader, a skull with crossed bones below it or something like that..), retry with 'fixmbr' ('fixboot' is another, but it should deal with partition records rather than bootloader itself). You need to get it run, or the only two options left are reinstallation of Windows or getting an image of Windows-only-bootloader MBR from your friend (466 first bytes, no partition-specific information, after which you still need to run something to get the partition records complete - fixboot at Recovery Console maybe? dunno - I don't use XP)
- now when Windows boots normally, and only that, you still have the Ubuntu partitions eating up disk space. Download and install some kind of partitioning program that can deal with ext3 format (that's what Ubuntu should use). Partition manager does, but it's commercial as far as I know. A Linux live-cd like the Ubuntu desktop cd does here just fine!
- launch the partitioning program that knows ext3 filesystem, or just boot the Ubuntu desktop install/live-cd like you normally would and once it's up, open Gparted from the menu - this works
- with the partitioning program select and remove the Linux (ext3 and swap) partitions. There should be one swap partition and at least one Linux partition
- now that the space is freed, either extend your Windows partition over them (you might want to run file system check on Windows first - scandisk, anyone?) or just create a new partition of type NTFS (XP and newer use this) or FAT32 (WinME,98 and such use this)
- reboot and enjoy
IN SHORT:
- reinstall Windows bootloader
- remove Linux partitions
- re-use the freed space by either extending your current partition(s) over it or create new partitions ("drives")
Note that you must deal with the bootloader FIRST and with partitions AFTER that. This is because if you first wipe out the Linux partitions, and use a Linux bootloader (that has some of it's stuff at /boot Linux partition), it won't find some of it's pieces anymore and refuses to work (you wouldn't like to do anything either, if you had your legs and arms cut off, right?). Therefore first overwrite MBR with a new bootloader - usually Windows' own bootloader, by using Windows' Recovery Console - and after that's working and you know you get your Windows boot normally, then reuse the space that Ubuntu reserved.
And if you used a third-party bootloader that has nothing to do with the Linux partitions you have, it's ok - forget about the bootloader stuff, just go to the step where you take up a partition manager program and deal with the partitions.
It's also possible to boot Linux with Windows bootloader (add Linux as a boot option to Windows boot menu), so in case you already are using XP's bootloader, don't "fix" it. It's a damn beast, and sometimes difficult to recover. Actually I would encourage to never touch it, if possible, because all it really causes is trouble