If you bought just a key or bought your copy of W7 off eBay then it may well have been a pirated copy. If the same key is used by numerous machines then Microsoft can invalidate it; sometimes it can read valid for months and then bam; often after it is too late to file an eBay claim.
It is also possible you received a false positive.
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.