The upgrade offer doesn't apply to XP.
If you don't mind the lack of security patches, and the resulting ever-increasing vulnerabilities, you can run XP literally for as long as you like. I know people who are still running Windows 95 and 98, and I have one friend who claims he has mission-critical software that only runs on Win 3.11.
Just be prepared for the day when you have to rebuild it. Make sure you have restorable image backups. Not just your original XP install disks -- those won't get you the Windows Updates. As of now Microsoft will still let you download Windows Updates for XP -- not any new ones, because there aren't any, but all the old ones.
But someday they'll take those offline. So be prepared and make full image backups of your system disk and every so often, get a spare disk and do a test "restore" to prove that your backup is still viable.
You'd be amazed how many people make backups but don't test them regularly to make sure they work.
Thanks for the tips. I've been in the computer industry as a software engineer since 1972. I've been around the block.
When forced to finally leave XP, I will switch to Linux.