Dual boot
Hi again, just skip to last para if this is too much info.
If you just formatted the first install and reinstalled, then only the new install will show up on your boot menu, your second install would still be there unchanged BUT, you would have to edit your boot ini file on the new iinstall to point to the second install and it would be accessible again, the boot ini file uses what is called an ARC Path, This is the way the boot manager will recognise where the operating systems are located on each hard drive.
Sorry to go into so much detail. but i will show you how it works here.
from my computer double click on your C: Drive then from the tools menu on the top; select: Tools> Folder options> View> uncheck the "hide protected operating system files" and check "show hidden files and folders" and apply at the bottom. close tha window, and when you look at contents of drive c: you will see the file file called boot.ini or just boot, there is also a boot.bak file which is the backup.
the contents of the file will look something like this

you should only have three entries though, put word wrap on in notepad to see entries more clearly.)
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(1)\WINDOWS
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect /tutag=bu4j60 /kernel=tukernel.exe
multi(0)disk(0)rdisk(1)partition(1)\WINDOWS="Windows XP Media Center Edition" /noexecute=optin /fastdetect
multi(0)disk(0)rdisk(1)partition(4)\WINDOWS="Microsoft Windows Longhorn" /NOEXECUTE=OPTIN /FASTDETECT /USENEWLOADER
multi(0)disk(0)rdisk(1)partition(2)\WINDOWS="Windows XP Professional x64 Edition" /FASTDETECT /NOEXECUTE=OPTOUT
c:\CMDCONS\BOOTSECT.DAT="Microsoft Windows Recovery Console" /CMDCONS
timeout is the countdown you see at bootime to select your operating system.
rdisk(0)partition(1) means to boot the operation system on first partition on the first harddrive (usually C
rdisk(1)partition(4) means to boot the operating system on the four'th partition on the second harddrive
So if you just format the first install you will loose the second entry pointing to your second install.
Alternatively! if your boot menu was ok then just copy the boot inifile on to your second install example: drag and drop it onto your drive D:, then when you format and reinstall just copy it back to your drive C: (easy as that)
if you get error when coppying it back to your drive c: then you need to right click the one on drive C: select properties and uncheck the read only button.
and hey presto when you reboot your operating system options are back.