For a period of 18 months, when you bought a computer from Comet, assuming it's a Windows PC (and 95% of those sold are just that), Comet offers a recovery disk for £14.99. The idea is that if your system fails then you can build it back up by reinstalling Windows. Comet has made and issued some 94,000 of these recovery disks.
Microsoft, however, does not believe these disks are legitimate. It regards them as pirate copies of Windows from which Comet has made around £1.4m in sales between March 2008 and December 2009, according to figures widely reported including in this Guardian article.
Comet's view, according to a statement, is that it regards the CD sets as a service the customer wants as Microsoft itself no longer supplies recovery disks.
The case is likely to hinge around Microsoft's contention that the disks were sold for Comet's economic benefit rather than purely as a service to customers, with no reference to Microsoft.