I run multiple boot on both the desktop machines at home. One is multi-boot for a range of flavours of Linux, the other is primarily XP Pro, but with a couple of Linux versions on another HD.
Regardless of which OS I boot into, they can all see the "other" OS's files, but they can't run the software on them. I did have to download some utilities to allow XP to read Linux file formats, and some versions of Linux can't read NTFS, but the principle is there.
The big advantage of multiple-boot is that it provides instant access if one of the other OSs plays up. It saved my bacon a while ago when the XP hard drive decided it had a bad sector in an important place (it was a hardware fault, not an XP fault).