Honestly, as long as you have a good antivirus and some common sense, XP should be just fine. A lot of newer programs don't support XP anymore, but thankfully Avast and Firefox still do, and they're both good programs.
Now, for specs, the MINIMUM I would recommend nowadays would be 4GB of ram and a half-decent dual-core CPU, like a Core 2 Duo or one of the Socket AM3 Athlons; older dual cores like the Pentium D or the Athlon 64 x2 are junk. Ideally you should be shooting for 8GB of ram and a more recent CPU, but at that point you'd have to have a more modern operating system to really take advantage. Windows 7 is a bit heavy on older systems, but overall I'd say it's one of the best operating systems out there for casual users, since it supports almost all the programs a normal user would want (Microsoft Word, iTunes, etc.), it has good hardware compatibility, it has a familiar user interface, and it's fairly stable.
Now myself, I use Manjaro Linux as my daily driver, because my machine has some problems with Windows 7, and I'm not a fan of the newer versions of Windows. It's a good OS, but you definitely need to have some technical expertise and experience to use it. It's also prone to bugs and quirks like any other Linux distribution, but the one thing I like about Linux is that if you have a problem with something, usually you can dig in there and fix it, as opposed to Windows where the only real solution to some things is to reinstall the OS.
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