Everyone who uses Ubuntu know how to remove Ubuntu when you installed it with wubi. But sometimes it's hard to remove (uninstall) Ubuntu when you installed it in separate partition (without wubi).
Some people may ask "why not just delete Ubuntu partitions?". No we can't do like this because Ubuntu installs a boot loader called GRUB into the Master Boot Record (MBR), if you delete Ubuntu partitions all the data will be deleted but not GRUB! So when your restart your PC, GRUB will look for Ubuntu , when it does not find Ubuntu it show some error and you cannot even boot into windows. So here I'm going to show you two ways to safely remove Linux.
First (Using MbrFix)
1. Download mbfix from here. http://download.cnet.com/MbrFix/3000-2094_4-10485990.html
2. Extract all files in mbrfix.zip to C:\.
3. Open Command Prompt (start->run-> type cmd -> OK).
4. Type cd \ and press enter.
5. Then type mbrfix /drive 0 fixmbr /yes and press enter.
6. Now you can delete Ubuntu partitions (/, swap etc) using Easeus Partition Manager or Windows Disk Management or other partition editor.
7. Restart your PC.
This will overwrite GRUB with Windows boot loader . So you will see Windows boot loader instead of GRUB.
Second (Using Windows CD)
1. Boot your PC with Windows XP CD and select Repair.
2. Then select the OS, once you are there you'll have a command line.
3. Type Administrator password. If Admin password is blank, just press Enter.
4. Type fixmbr, and then press Enter.
5. Restart PC.
6. Delete Ubuntu Partitions.
1. Boot PC with Windows Vista installation disc.
2. Select a language, a time, a currency, a keyboard or an input method, and then click Next.
3. Click Repair your computer.
4. Click the operating system that you want to repair, and then click Next.
5. In the System Recovery Options dialog box, click Command Prompt.
6. Type Bootrec.exe, and then press Enter.
7. Delete Ubuntu Partitions.
This will also overwrite GRUB with Windows boot loader.
(i) If you have any doubts about using Recovery Console read this and this.
(ii) It's always good to back up MBR. (Raymond had written an excellent article about that.)