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.
My 'old' laptop is running Win 2000 Pro, which was part of a botched 'repair job'. I have the recovery discs for that LT, as well as for this "newer" one. I've been getting by, as I use the 'old' one for my PT instructor time.
I've tried in vain to install the rescue discs, but to no avail. I guess it's because of the Win 2000 Pro being the OS.
It would be nice to install the DSL, restore the DVD to record, and .....run a little quicker.
I may have a windoze 2000 disk if you have the COA sticker with the secret code. If I don't I know my neighbor does. I got it on Ebay, still in the shrinkwrap for $10. These things are kicking around in desk drawers, never opened and they pop up on Ebay regularly. You can also get old business software cheap that way. As long as it is still a sealed package, it is legal. With MS operating systems you don't get the secret code with a disk tho so you need the COA sticker.
Greg: What I meant to say above is that (older) LT came with XP from day 1, which is on the rescue disks (Toshiba). The botched repair wound up with 2000Pro, which is not friendly with the DSL, DVD, etc.
Think I should just shop for an XP disk? Or is there any way to use the recovery disks?
The school district I used to work at, and still keep in touch with has over 2,000 workstations on XP and they have no plans of upgrading anytime soon. They stayed on Win95 until around 2001/2002 and didn't upgrade to XP until around 2004. When I got laid off, they still had around 200 machines running Win2000, but they have since replaced them with more XP machines.
The real answer depends on what you want to do; basic stuff should be fine, but if you have to keep up with the bells & whistles on the Internet you may discover that software support will start to drop off. But, with the huge numbers of people still using XP it could be a while. It sounds like MS is going to drop off XP support for its software sooner rather than later; probably trying to "force" people to upgrade, though third-party software developers will probably still be supporting XP for a long time.