Because I was unable to uninstall Office Ultimate 2007 via Add/Remove, I did it manually following the instructions on the microsoft website, however, during the installation some difficulties occured and not all the files in the steps were totally deleted, but I managed to get to the end of the steps. Is there any way of checking that the program is comletely off my system?

Remove Programs
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Uninstall Programs Manually

Just because Windows XP has the Add/Remove Programs feature it doesn't mean your application will appear in the list. Furthermore, even if it does appear, it's no guarantee that the uninstall feature will work. When you run across one of these situations the items listed below will help in getting rid of the application. Be aware that these steps may not remove everything associated with the application and can impact other applications on the computer. Have a backup or restore point and use caution.

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Find the directory for the application and delete all the files in the directory. Delete the directory. e.g. click C: under My Computer then Program Files etc.
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Open regedit (start/run regedit) and navigate to HKEY_LOCAL_MACHINE\SOFTWARE and find the folder for the application. Delete the folder.
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Open regedit and navigate to HKEY_CURRENT_USER\SOFTWARE and find the folder for the application. Delete the folder.
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To remove the application entry from Add/Remove Programs (if present) open regedit and navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\Uninstall and find the folder for the application. Delete the folder.
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Some applications have Services attached to them. If this is the case, navigate to HKEY_LOCAL_MACHINE\System\CurrentControlSet\Servic es, locate and delete the service.
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In Windows Explorer, navigate to the individual user settings and delete program references. Common places to check would be:

C:\Documents and Settings\All Users\Start Menu\Programs and delete relevant entries.
C:\Documents and Settings\All Users\Start Menu\Programs\Startup and delete relevant entries.
C:\Documents and Settings\%YourUserID%\Start Menu\Programs and delete relevant entries.
[Do this for each User ID listed]
C:\Documents and Settings\%YourUserID%\Start Menu\Programs\Startup and delete relevant entries.
[Do this for each User ID listed]

If no entries were found in the previous step and the application launches automatically, navigate to
HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Windows
and delete the entry.

C:\Program Files\Microsoft Office\Office12 is not deleting, as there are .dll files relating to MS Groove. It says I need permission to perform the action, though I am the only user and administrator of this laptop.
When I do Shift+Del, then it starts to delete but then it says "The action cannot be completed because the file is open in another program." I tried using Killbox to delete the folder but it says the files cannot be deleted.

I also tried in safe mode but it still didn't work.

If you are sure you want to remove it try this:

Download MoveOnBoot 1.95 - Allows you to copy, moves or delete files on the next system boot - Softpedia

It didn't work, there was an error.

What exactly didn't work? What exactly was the error?

When I press "Start", it says Access is Denied.

Sorry I am not following.

When you press start where?

Are you talking about Move On Boot?

Have you downloaded it?

Has it installed?

Are you saying you cannot run it?

Haha. When I press "Start" to deleting the first .dll file in the Office12 folder (GrooveMisc.dll)
Yes.
Yes.
Yes.
No, I can run it

See if anything here helps:

Troubleshoot "access denied" when opening files or folders - Windows Vista Help