One of my friend today asked me about removing Linux partitions & GRUB (from a dual boot system) and return back to windows alone. Removing Linux involves just formatting/removing the partitions. Now to remove GRUB either do fdisk /mbr from a Windows 98 bootable CD or do fixmbr after booting into repair mode with Windows XP CD. But if you have none then to remove GRUB you will need some utility like this one and if you reboot before doing that it might make GRUB unable to boot into Windows. It will get stuck at GRUB> prompt only. So there is an option: to manually boot the OS you want (ie Windows). A quick search gave link to this thread. It involves few commands on the GRUB prompt:

grub> rootnoverify (hd0,0)
grub> makeactive
grub> chainloader +1
grub> boot

It will load the NTLDR where your Windows is installed in Partition 1 on HDD 1.