Scroll down a bit.
Find the similar label on the outside of your computer.
Tell us what the specific model number is - that's at the end of the first line
Tell us it's Product Number - that's on the third line.
Older Compaq computers may have a product number with nine characters - xxxxxx-xxx
I don't know of any situation where you wouldn't be able to see the partition C was on except this one -
If you have a desktop computer that has two physical hard drives, the one C and Windows is on is a SATA drive, the one D is on is an IDE drive, if the mboard's bios has the SATA controllers in SATA a.k.a. AHCI mode, the initial files loaded by the CD will not detect a SATA drive by default because the CD has no built in SATA controller drivers.
If you set the bios so that the SATA controllers are in some sort of an IDE compatible mode, the initial files loaded by the XP CD will detect SATA drives, as IDE compatible drives.
Whatever that setting is, for a brand name system bios version, I or we probably can't tell you where and what it is, because usually we cannot find enough info, or we find no info at all, about what is seen in the brand name system's Bios Setup, but whatever it is, it's set to SATA or AHCI now, it's changable, and at least one other choice is an IDE compatible setting of some sort.
Once you can see both the original C and D partitions, you're probably still not going to have enough space to install Windows.
If there is a lot of free space on C, or D, you could make that partition smaller and possibly make a partition large enough for Windows to be installed on, on the unallocated space made available by doing that, but you CANNOT do that with what's built into the CD's contents or in Windows XP or below without losing all the data presently on C, or D, whichever one you need to make smaller.
However, you CAN do that with a third party "partition manipulation" program.
E.g. the freeware Easeus Partition Master Home Edition.
By the way, the second installation of XP WILL NOT see it's Windows partition as having been assigned the C drive letter.
If you want it to see it's Windows partition as having been assigned the C drive letter, you can have the third party "partition manipulation" program HIDE all other hard drive partitions that have already been assigned logical drive letters BEFORE you run Setup for the second Windows installation - after Setup has finished, you can un-hide those partitions again.
When you dual boot, when you boot the first Windows installation, it will see it's Windows partition as having been assigned the C drive letter
; when you boot the second Windows installation, it will see IT'S Windows partition as having been assigned the C drive letter
E.g. the freeware Easeus Partition Master Home Edition. Once it has been installed in Windows, I believe you can make a bootable CD with it that you can boot the computer with and use that program to HIDE all the existing hard drive partitions that have already been assigned logical drive letters.