In terms of newer software, or hardware?

In terms of hardware, if the machines are fast enough for you, and you don't mind the inflated heat and electrical usage (which was particular to the Pentium 4 generation of Intel processors, not to older machines in general), and it runs the software you want/need, there's nothing wrong with it.

In terms of software, if you're using it off-internet, and you're satisfied with the user experience, great. Sufficiently old software should NEVER be connected to the internet directly or used to browse any but the safest sites on the public internet -- both put not only your machine but other people's at risk.

There's also the question of user experience. I find XP to be a very dated pain in the neck, and unlike some of the simpler Linux UIs (which have the benefit of speed and simplicity to make up for their limits compared to say Cinnamon or KDE) doesn't even have reliability or speed speaking for it.

Back when I used XP, reinstalling Windows because something corrupted itself was pretty much an every-6-months activity. Vista or 7 are MUCH better at maintaining itself, and firewalling applications from one another such that the OS and/or applications don't get corrupted. They also finally started catching up to real multiuser OSes on security and the ability to safely share the machine with other people, which is irrelevant on a true single-user machine but very helpful if you ever share them.

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IMO, speed is fairly generational. A 2ghz first-generation Core 2 Duo or 3ghz Pentium D is pretty much the slowest machine that's been tolerable with generation of software that's been out for about the 6-7 years those machine have been out. Those were fairly high-end machines at the time; they're now a little faster than the very lowest-end new machines.

At the same time, the very fastest single-core machines available at the same (3.8ghz? P4-based Xeons) I generally find intolerable. We've still got a few kicking around at work, and if you put two physical processors in them they become tolerable -- although they'll be pulling over 200W just for the CPUs.

In the past 6-7 years, memory capacities needed have gone up (2gb is no longer really fine) but the basic processor speed needed hasn't changed much for the OS or for browsers, or for any of the other common apps.

Probably the very low hundreds.

A 5-year old computer is generally fine today; that puts you in 2008, and unless it was an incredibly low-end one at the time, you're looking at a dual-core system, a modern processor, and at least 2gb of RAM.

I make my living writing software; back in '09, when we were looking at new hardware, we benchmarked the old hardware vs. new. Doing a regular clean build (not even the most onerous sort, with libraries and code generation) had gotten up to about 15+ minutes on the old hardware -- on Linux, worse on Windows thanks to the weaker filesystem and antivirus. The new hardware was just over 5. We would typically pull and do a clean build on our own systems once daily, maybe twice if there was a really big change.

If we call a developer's time worth $40/hour (probably a bit too low on average base salary, and way too low fully burdened), and ignore all the other time savings/productivity benefits to the speedup (or the morale benefits!), 10 minutes a day over a 180-workday year (after holidays, vacation, meetings days, working from home, etc) equals 30 hours waiting on the machine in the first year or about $1200.

Add in the other benefits, and the slightly-over-$2000 machines we were getting at the time were probably paying for themselves in the first year.

For desktops, the newer even faster machines we started getting last fall (like many companies, we're on a 3-year depreciation cycle) are about $1300 now. The speed difference, lacking a big generational bump, are incrementally faster (a bit under 50%) rather than 3x faster, and I don't know anyone who much cares whether they have the new machines or the 2009 vintage ones. (We could buy 8-core machines now, but they're not cost effective and lose quite a bit of peak single-core speed relative to the fastest quads.)

For laptops, back in 2009, we couldn't get a laptop anyone wanted to do serious development work on our product at any price, from any manufacturer, and the closest you could come weighed a ton and resembled a space heater. When the first decent laptop processors for our purposes came out in early 2011, it was a really big deal.

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For general, non-development use, I have a couple of Lenovo T200s at home which were being e-wasted at work. They're about the slowest processor I'd recommend to anyone (2nd-generation 1.86ghz Core 2 Duo), but they're perfectly usable even for my impatient self, have great battery life, and are super light and portable. Add in a cheapie $100 SSD, and you've got something very competitive with a brand new ultrabook.