If you want to 'hide' one OS from another in a Windows managed dual boot; once both are installed ...
In Windows 7 remove the XP drive letter.
In XP remove the Windows 7 drive letter, if Windows 7 is the "System" partition XP may not let you remove its drive letter.
The real way to 'hide' one OS/HDD from another is to install each OS to its own Hard Disk Drive (HDD) one at a time.
Start by disconnecting all HDDs except the one to install Windows 7 to from the motherboard; when that's installed, disconnect that one and connect a second HDD to install XP to; then power down the PC and connect both HDDs; then set the OS/HDD you want as default in the BIOS, then when you want to boot/start the second OS/HDD use the BIOS one-time boot menu specific to your mobo to make the selection.
Asus - F8
HP/Compaq - Esc
Sony - F2
Acer - F2
Gateway - F10
eMachnes - F10
Toshiba - F12
Dell - F12
IBM/Lenovo - the blue Thinkvantage button