I have three partitions in my HD, a NTFS partition of 50GB, a Swap partition of 1.5 GB and the ext3 partition with 55GB, I want to delete my ext3 partition to resize it and split into 2 partitions, but when I delete any partition, grub gives me an error at startup, and I have to use my windows disc to fix the MBR.
Right now, grub at startup shows me 4 options:

Ubuntu,2.6.15.3-adrian
Ubuntu,2.6.15.3-adrian (recovery mode)
memtest+86
Windows XP Professional

So is there a way I can delete the ext3 partition, erasing Ubuntu and make grub work normally showing me only the Windows XP Professional option?

The biggest problem is that you're deleting the ext3 partition, so you're deleting all of grub's configuration info. If want to use grub full time for your boot loader, make a separate /boot partition. Then when you delete the linux partition, grub's config files will remain intact on the /boot partition.