Last bit of spam from me on this.
If you just want to poke around, use the linux live CD and see if it's worth trying to rescue. If you want to play with the server OS, you can try to find the serial. I guess the next step is to find a pal with access to microsoft's volume licensing site who has access to Win2K3 SBS. We never used that here at work so we only have access to Win2K3, Win2K3 x64, Win2K3 R2, and Win2K3 R2 x64. And Win2K8 and later stuff, but that's nowhere near Win2K3 SBS. Since you don't have any experience with a server OS, it might be fun to play around with it if you can get the serial and find a source for the ISO from MS.
Now, something else to consider is you still won't be licensed for Win2K3 SBS. It's possible that license was repurposed by the people who auctioned off the system and legally you're not entitled to it. The HP workstations I got had a Vista Business tag on the outside so I had the serials there and they were OEM licensed for the box. You're probably not so lucky. However, that OS is oh so old so I doubt anyone cares one way or the other, if it's even still being used by the auctioning company. Hell, for all you know, they went bankrupt and these assets got liquidated.
Crap, I only just now read "and I do have the product key for the current OS on it" and had it register in my brain-meats. Hell yeah, find yourself an ISO, then! And/or check that that product key matches what is actually being used for the installed OS.