Basically, if your application doesn't change, your software doesn't need to change. In a business environment that may just be your tax program.
You can go on forever using an OS after the official end of support as long as it supports the software you want to use. This only affects them releasing new patches and getting help from the vendor. If you have bundled software (pre-installed) microsoft probably won't help you anyway unless you pay them. You end up talking to Dell's (or HPs) Bob from Bombay who will be reading from a script. The other issue is drivers for hardware. That axe swings both way. Old hardware may not be supported on a new OS and new hardware may not be supported on an old OS. You still may be able to find a driver that works but you are on your own.
The flip side of that is old software is usually a lot more stable than new software. I still use DOS for a lot of things.
The official policy from microsoft is that they support an OS for 24 months after the last service pack is released. They have made an exception with XP since that would have been 4/21/10 (SP3). They have put a stick in the dirt and said the OEMs have to stop pre-loading XP home by October. I am not sure if that means support stops in October or October 2012. I have heard that XP is the most used OS in the world right now. Microsoft has a huge installed customer base that is not going away until they buy a new machine. The good news is XP is pretty stable.
Personally I think there are great deals right now on used P4 class machines from name brand vendors, with XP Pro stickers on them (licenses). That is really the only reason I am migrating the fleet to XP. It is a good idea to download the SP3 package from microsoft and put it on a CD so you can reload it if you need it. Also make sure you have a good restore disk for your machines with all the drivers. Those "free" driver web sites are becoming less free and a lot more cumbersome to use. I also worry about getting a driver with a little something "extra" in it when you go to a no name site. I make it a habit to save driver while they are still available from the manufacturer's site.
As for the applications, there is always oldversion.com to get older versions of commonly used software. I just went there the other day to get Firefox and Flash that would run on a W98 machine I am building (jukebox with weather radar on a second monitor for the tikibar)
That is DOS MPXPLAY.EXE and Firefox 2.<sumpin> running on W98.
