Probably first of all you need a Professional version of an OS to allow two OSs on the computer. And 25Gb for Windows 7 is not really going to work since it stores an additional copy of the OS usually with the OS in case something goes wrong with it (well on my computer it does) so you need probably more like 70Gb to be safe, maybe less. I don't have much on my C:\ partition which is the primary partition and the active partition that starts up the computer and loads the OS in the first place.
The formatting in XP may be different than Windows 7 which has to be NTFS (new technology file system). Well I would have to check my old XP computer to know what I have in XP anymore, but with XP I could also have Fat32 formatting or even the original Fat16 formatting for old ms-dos games which I do have on my XP computer. Fat32 is 32 bit formatting (like Windows 98) and Fat16 is ms-dos or 16 bit formatting like Windows 95 or ms-dos.
Now it may be different for Windows 7 if you have a DVD with the OS on it, but usually on newer computer they do not include a DVD anymore with the OS - they have another copy of the OS on the harddisk like mentioned above if anything goes wrong with the OS and recover DVDs you have to make on your own, or even DVDs with the OS on it on your own. You do not get your own DVDs with the OS on it anymore usually and the manufacturer of the computer would have its own program on the computer for all of that - or you can make a backup in Windows 7 or the harddisk partition (for the OS) or buy a program that does that kind of backup and all of that kind of stuff to insure that you do not lose your OS. You usually should have back up on another harddrive because if the original harddisk drive fails, then when you put in a new harddrive, you can re-install the OS from the backup or from the DVDs you made. That is just the way they do it now, and even if not, then the manufacturer may have a cloud kept version of the OS you can download from them again, if you have your computer registered and all of that stuff.
Way back when Windows 2K was out, the only way back then to have two OSs on a computer is one of the versions had to be the Professional Edition of the OS so two OSs could be installed. I have not done that since so I do not really know how that has changed or if it has changed.
You should really do some searching on the Internet and put in things you do not know and read about it, even perhaps at the manufacturer's website perhaps, but there is a lot of info out there, just make sure to find the correct info on computers - and on forums for the OS and whatever.
Also like mentioned up above for the formatting, I think that Windows 7 can read a Fat32 partition, but you will need third-party software to format any partition that way, because the only formatting Windows 7 (or Vista maybe I do not know) is NTFS type formatting. XP may be that way also, but other formats could be done in XP but not in Windows 7 without third-party software.
http://www.partitionwizard.com/free-partition-manager.htmlor perhaps Partition Magic (which I use to use but then - you would have to search for that software if made anymore).
Two different programs.
And no doubt that is why XP can not see one of your disk because you have changed your computer and now the OS is lost as to what you are doing probably. But do a search for any info you need and really find out.
I have not tried everything so I am just going by the past and what I did on my old, old computer not used anymore with Windows 2K and having two OSs on a computer.
If XP was the professional version of the OS then what you did should work and you should have a choice as to which OS you boot up the computer with, but if it was not, then Windows 7 should have taken over and that would be the only OS you can boot up the computer with. As to why it would not see the harddisk then if you had the professional version of the OS then the formatting of the harddrive probably has something to do with it then.
( and the type of formatting on the harddisk).
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