I have to question the relevance in the same way everyone else is. The other software you're using may be far more at risk. Windows XP can now be considered mature so even if it is not updated it still may be more secure than a more modern Windows with all of its new bugs. Plus, in terms of software support, virus authors are just as likely to be dropping support for an OS that few people might be running.
While you are correct that 3rd party software is the most likely cause of problems even on XP, your assertion that it could be more secure than newer versions is false. Vista and up have two very important security features that XP lacks: Not running as admin by default and ASLR. Those two features make security holes in 3rd party apps harder to exploit. XP can be run as non admin, but it is harder because it doesn't have any OS hooks to help poorly behaved applications.
Don't expect AV software to help, it is greatly overrated. Knowing what you are doing and having a secure OS & browser will be far more effective in stopping viruses than any AV.
Besides the security updates, if you have a reasonably modern PC (Core 2 and up) with enough RAM 7 and 8/8.1 are faster than XP. They even run well on even slower PCs if the HD isn't terrible and the video card is at least DX9. If you have more than 3 GB of RAM than the 64 bit versions of 7 or 8 are obvious choices over XP.