If it isn't broke.... it will still function when Micro$oft stops support in a couple of months.
Does it still do the things you need it to do? Use it till the wheels fall off. I've got 4 boxes running XP at the moment. I'm not planning on doing anything with them. I ran OS/2 for many years after support was dropped for it. A corrupted backup was what finally made me decide to go to the dark side.
Backup - get an external hard drive for $50 and back up the data you want to keep off it. A 2005 machine probably doesn't have a large hard drive in it.
If you replace the machine, you'll be into Windows 8. Get the freeware "Classic Shell" and you can have a start menu that looks like XP and you'll shorten the learning curve. Doug