windows 7 - How to re-partition old XP boot drive (with OEM partition) after installing Win7 on new SSD?

Old situation: Machine had 2 232 GB HDDs:
- HD 0 contains a 70 MB OEM partition and a 232 GB Windows XP boot partition
- HD 1 has one partition (data disk)

I bought a new SSD, added it to my machine, let Windows 7 install on the SSD.
I then had a boot menu and could boot into either Win7 (default) or Win XP.

Now I want to re-use HD 0 as one data partition.
I thought it would be as simple as going into disk management, kill those 2 partitions on HD 0, make a new one. Here's the disk management info:

As you can see the 'Remove volume' option is grayed out.

I tried/checked:

That last step was a mistake, now my computer would not boot. From a boot disk I could set it active again using diskpart; so now I'm back to square 1:

How can I re-use the entire HD 0 as a data disk?
(It would be OK if I could just format it and leave that little OEM partition, but format is also disabled)

My situation seems similar to this post without answers.

My boot sequence is 1) Floppy 2) Onboard CD 3) Onboard SATA.
My SATA controllers (BIOS setting) have the HDDs on 0 (the D: HDD disk 1) and 2 (the E: HDD disk 0), the SSD disk 2 on 3. All AHCI BIOS.