In all my life, with all of my computers, XP was my most beloved O/S. It never complained, it never gave me a moment of grief, it did exactly what it promised to do and it was a workhorse.
One of my recent purchases was an old Dell Optiplex with a clean install of Windows XP Pro 32-bit. Now, I like Windows 7 enough, but it has issues; one of which is spotty support for older games. As I have 64-bit W7 I can't install any game that uses a 16-bit installer even if the game itself is 32-bit. I've also had some odd issues with certain games- controls in Simcity 3000 were too sensitive and unworkable, Shadow Man suffered some strange physics glitches(sliding on sloped surfaces) and some enemies couldn't be killed which made the game unbeatable. I also thought I could use a second Windows PC, given the issues I've had with the Gateway which is why I went with a second PC instead of installing XP onto my Gateway.
After some searching on eBay for what was available I settled on a refurbished Dell Optiplex 745. 80 GB HDD, 2 GB RAM, 3 GB Pentium D processor. Nothing special, but it does have SATA ports so I can at least replace the DVD-ROM or HDD easier if need be. Wonderful case design though- it's made to sit flat like an old 80s PC so I could put it above the TV, and the top panel pops off easily with a spring-loaded latch, unlike the brute force needed to get the side panel off some cases. I can upgrade if I wish, being older may mean stuff like a PSU or video card could be cheaper and I can boost the RAM for cheap, most likely.
Right now I'm loading up and playing games I haven't been able to touch in 4 years. Shadow Man is playable without the glitches, I can run Simcity 2000 and 3000. I can install Max Payne 1 and Kiss Psycho Circus along with a pile of other old games. Not sure about a web browser- the current build of Firefox may work(it eats up about 400 MB of RAM on my Gateway PC with plugins using much of that) although I plan to take this online only if necessary.
My total cost was $65 shipped for the PC with keyboard and mouse, plus the cost of a 3.5mm audio cable and a USB extension cable, so not a great expense- I recall back in 2002, my Compaq PC was around $700 total. I actually would advise people to own at least 2 computers given the chance of malware or hardware failure to make one inoperable- granted, a lot of people now own a PC/Mac, laptop, pad, phone, whatever, but even if you aren't it's still worth looking into something used/refurbished for cheap as a backup. Having an older OS on hand is great for retro gaming seeing as newer Windows won't handle everything and DOSbox isn't 100% reliable for those purposes, and you're likely to hate some features of a newer OS. Most people loved XP, almost everyone hated Vista and 7 was better loved than Vista but nowhere near XP's level of regard. Seems 8 is also not winning people over.