There is no need to upgrade unless your computer crashes.

If you are connected to the Internet, you should be wanting to have your system's security upgrades. At some point, these will no longer be supported for an older operating system.

If you get other new hardware, eg. a printer, you might find drivers are not available for your older operating system.

My main Windows usage stopped with Win 2000 which I didn't think was bad. I did get Vista Home Basic on a new laptop. I've only played with it briefly and I think it's cut down and that Win 2000 was better. Vista's only function for me at the moment is to allow me to check a website works with IE7.