BeOS is an operating system designed by Be Inc as a powerful multimedia desktop OS. Originally, it was intended only for their own computer system that had its own A/V inputs and outputs, known as the BeBox. Eventually it was ported to the Macintosh, and later then to the PC.
This is the PC "Personal Edition", which I believe was offered at no charge (it certainly is now) and can be run under just about any flavor of Windows (of the time, anyways), but only in a dual-boot configuration. The "Professional" version installed from CD as was normal for an operating system of the time.
On Win9x systems, you can start it by clicking on the desktop icon the installer creates. On NT-based systems a boot floppy is required but easy to create.

Unfortunately, although Be had a solid product in BeOS, they couldn't compete with Microsoft or Apple, the two market giants. The Macintosh ultimately had a stranglehold on the multimedia market, and Microsoft of course had a near monopoly in the commodity PC market, and chose to leverage that by making an agreement with OEMs forbidding them from including alternate OS and shell software on their Windows machines.
Be went out of business shortly after "changing focus" to internet appliances in 2001.

Fun fact: BeOS was considered briefly as a replacement for the original MacOS and a basis for Mac OS X. Ultimately that deal fell through and Apple bought NeXT instead, and the rest is history. But imagine if Apple bought Be instead of NeXT!

The BeOS splash screen.

And here is what we get upon the initial boot. Unfortunately, these screenshots will have to be in greyscale only because BeOS doesn't seem to support the S3 graphics virtualized in Virtual PC.

This VM wasn't configured for networking, but I'd imagine just about every link in that welcome document no longer works as Be's site is long gone.

This is the default desktop. BeOS has a vague UNIX feel to it, except without all the multi-user and security stuff. Most of the default icons link to webpages that no longer exist. The "Personal Edition" icon links to the root of the "virtual drive" that BeOS runs from.