darkorbit, you are not fully responding to the questions, which may make it more difficult for members to help you.
Here is what is known based on the information you supplied: You have one HDD with two partitions, one C: with WinXP Home, and one J: with WinXP Pro. A WinXP managed dual-boot exists such that relevant boot information is contained in, and controlled by, the boot sector of partition C:. We do not know what else you have on either partition.
As Nodsu stated, there are a "zilllion" pointers to J: in the registry of partition J:, so you cannot just switch J: to C:. Additionally, as I indicated in a prior post, you cannot completely wipe-out the C: partition because you will lose the boot information. Therefore, the options I see that you have are:
1) Delete WinXP Home, without deleting the boot sector. Programs installed on the C: partition will be useless and must be reinstalled on J:. Data files on C:, if you wish to retain them should be copied to folders you set up on J:. You will most likely have to edit the boot.ini file to remove the path to XP home. You will now end up with a partition (C
which has nothing in it except the boot information for J:. You will need 3rd party partitioning software if you want to non-destructively expand the J: partition and correspondingly shrink the "empty" C: partition, or
2) Copy all data you wish to retain in J: to folders you set up on C:, then backup all programs and data you wish to retain on C:, repartition the HDD into one NTFS partition, format, clean install WinXP Pro, the HDD will be assigned the drive letter C:, restore programs and data from the old C:, and reinstall the programs you want to retain which you had on J:.
The lesson to be learned here is that it is easy to get into a WinXP managed dual-boot, but a nightmare to get out.
Good luck.