Dual Boot with XP/Vista

I have read many threads on this topic but never one that was quite like my senerio.

I have a computer with two drives. Both are IDE. The second drive is not important as it is only a place for pictures and video. The first drive is where XP and Vista reside.

The first drive is partitioned into two volumes. I installed XP on this drive in the first partition. The drive was clean before this. I then installed Vista on the second partition of the first drive. Everything works fine when it boots asking you what OS you want and loading the default after "X" amount of seconds.

What I want to do is get rid of the XP partition and use the Expand/shink feature of Vista to combine the space back with the Vista partition as my drive is only 40 gigs that houses both OS and I have no need for XP anymore. Here lies the problem... Partition 1 has the "Active/MBR" according to the "drives manager" when you right click the computer icon.

How do you remove XP, merge (expand in vista I think) newly alocated space back with the second partition which vista resides on and still boot into the OS Vista? Is this even possible?