As far as my understanding (which is basic at best) goes, Windows can't read the GRUB (or lilo or the other bootloaders) or the linux file system so it will pretend it is not there ... If your gonna set it up to dual boot again, Windows will have to be installed BEFORE
Linux is. and if you are just going to setup Windows with no more linux boot, you will have to re-format the drive in a way windows can read (I think that is NTSF) ... So deleting the partition, I think you would do by loading a LiveCD, do fdisk, after that you have to find and delete your partitions (I think fdsik -p but not 100% on that), Sorry but I do not know the exact steps ... But I know for windows to be installed (either alone or dual) Linux must go, after that just put the Windows disc in and it should 'see' the Hard drive.
Your VERY first step will be to save all your important files, no matter what you are doing.
I'm gonna go search google and see if I can find the exact steps. Good luck!
EDIT: Links for the process ... Sorry all I can do is guide you else where, but I've only installed Linux to take the place of windows ... Not the other way around.
How to Remove Linux and Install Windows XP
I assume you are going to win7, But the steps should be the same. This link is from Microsoft Support.
Not a link, cause it is short enough to post here
from Yahoo Answers!
"Boot the Ubuntu LiveCD, find the program gparted and run it. When it opens, click on each partition and delete it. Once they are all deleted, click on the partition box and make it NTSF. When finished reboot and insert your Windows 7 disc. It should find the partition. You should use Windows 7 to redo the partition so it will be correct."
These are the only really concise ones I can find. Some sites say that it should be able to just install from DVD, Others claim linux does weird things sometimes (such as hide the drive) haha. Hope these links help.