err....
If you lost only the media (CD-ROM) but you still have CD-KEY, you can burn a new cd from any other cd media (of the same kind, OEM--> OEM, VLK--> VLK, Retail---> Retail, etc..) since the CD-KEY (that is the proof of license, not the media) is NOT attached to the media.
It is NOT considered a piracy act (as MS told me when i asked the exactly same stuff in 2004).
When asked by MS you should present CD-KEY and not the media.