I used to have a system on which I used to reinstall XP as and when needed when I thought that there was a problem. It sometimes ranged at every 2weeks and varied upto 3months.
However, on another system (my brother's), where all the compenents were imported (which I don't think makes a difference), and since I dont' have the authority to reinstall his system, it's been working quite decently for about 2 years now. It all depends on your mindset. If you see slight problems which can be sorted out with a bit of tinkering here and there, then you really don't need to reinstall. But for most of us, since we are all so used to reinstalling, we don't put in the effort or time to solve such issues and take the easy way out! which is reinstalling.
I have personally challenged myself several times to try fixing major winxp crashes without the need to reinstall and have succeeded a couple of times. One thing that I would suggest is that you always have a backup copy of your major boot files like boot.ini etc. on another disk. And the one thing that has saved me so much trouble so many times have been the good old
Win98 Startup Floppy Disk. It has worked several wonders for me at times of desperate help!