Keep your hardware, upgrade to Windows 7 or 8
Depending how ancient your XP system is, you might be able to circumvent security uncertainties by simply upgrading to Windows 7 or Windows 8.
Windows 7 offers a more natural upgrade path for XP users; it's not exactly the same, but if you're a longtime XP user, you'll feel more at home on the Windows 7 desktop than anywhere else. Most crucially, it has a Start menu and no Live Tiles. Unfortunately, Microsoft no longer sells standalone Windows 7 licenses, which means if you don't have one, you'll need to buy an OEM license. They're available online but do not include typical customer support.
XP-to-Windows 8.1 upgrades present other challenges. A lot of XP machines lack the specs to run Windows 8.1. Even if they can, the experience will be sub-optimal.
If your computer is up to snuff, 8.1 is much more usable than its poor reputation might imply. Yes, it still has quirks, and no, the Live Tiles aren't very useful on old PCs that lack touchscreens. But thanks to the improvements introduced in Windows 8.1, the UI can be configured to function more or less like a fast, more-secure version of Windows 7, minus the Start menu. This flexibility makes 8.1 much more practical for non-touch equipment than the original version, and Microsoft has already confirmed that the non-touch experience will receive more attention in an upcoming update, including changes that might make the tiled Start screen more palatable to mouse-and-keyboard users.