All jokes and blame aside, this is being heavily exploited and is condition red/severity A across every major security vendor in the world. If you think you are safe by not patching your machine, you're in for a rude awakening.
If you think you're safe using an operating system older than Vista, you're really just biding your time until you lose everything.
This vulnerability is in the SMB (Server Message Block) framework and is very easy to exploit.
Please do not become a statistic!
Thanks to rarestia for the ping!
Interesting: Microsoft no longer supports Vista, but they released a Vista patch for this exploit. Nothing for XP.