In-Memory File System for Windows

You should also consider whether you really need this (premature optimization, yadda, yadda). On all modern operating systems, filesystem I/O is cached anyway, so frequently-used files are essentially as fast as a RAM disk.

Related question (with many good answers): RAM drive for compiling - is there such a thing?

Commons VFS provides handy interfaces to virtual filesystems, inclunding in-memory file system.