Lost C drive

My c drive is invisible in explore my computer, how is this possible? The only way I can get it to reveal is by opening my computer and going to manage. It does reveal the c drive, but only temporarily and out of order from the other drives. This has been going on for a while and nobody has been able to tell me why it disappeared. Only beginner to intermediate savy on computer problems. Also, every time I go to open Word, Excel, etc the Windows installer engages and wants me to pop in the Windows Xp Office Professional Disk to load the program. When I do so it doesn’t find and I can’t find the appropriate program to reload in part because it is invisible on the invisible c drive. HELP! I’m running Windows XP Home Edition with 3rd updates