There are tons of awesome programs for Windows XP. XP is the last version of Windows which works well out of the box with a lot of older devices and classic programs designed for Win3.1 and the 9x series, so you've got quite a wide range of software to work with. What I'm using right now:
-Mozilla Firefox 26.0 (the first version that works properly with the Twitter desktop page and a lot of other modern websites; you can go up to 52.9.1ESR, but I don't need it now)
-VLC Media Player 0.8.5 (the first version that allows me to properly stream .3gp videos with no trouble; the latest version still works, but I'd go no higher than 1.1.11)
-CCleaner 2.36 (the last version that will work with Windows 98/98SE/ME; it's lightweight and lightning fast, and my preferred version because of its simple/effective interface; AFAIK the latest version still works but I'd go no higher than version 5.32)
-Defraggler 1.21 (the last in the 1.x series; provides a fast and reliable defrag without unnecessary bells and whistles that the newer versions have)
-Disk Cleaner 1.5.7 (the last version of this obscure yet useful program that will work on Windows 9x/NT4/2K; you can go up to 1.8.1795 [which is the latest/presumably last version], but right now I don't need it)
-MS Office 97 (takes care of all my document/spreadsheet/presentation needs perfectly; you can go up to Office 2010, but I don't need it now, and I'd advise going no higher than Office 2003 w/the compatibility pack)
-Opera 9.27 (the last version with the classic 'Technicolor' skin; still very good for older/basic HTML sites, and while you can go up to 12.18 [if you'd rather stick with Presto Opera] I find myself not needing it at the moment)
-Winamp 2.79 (the oldest version I've tried that works reliably for my needs on a daily basis; the latest version [5.8] still works with XP, but it offers no major advantages to me over 2.79)
-7Zip 9.20 (the last version natively compatible with Win9x; the latest version [18.05] still works with XP, but I don't find myself needing it now)
-WinRAR 3.51 (very fast/reliable, and will run even on Windows 95; the latest version [5.61 IIRC] still works with XP, but I'd go no higher than 5.40 because of later versions defaulting to RAR5 [which is not compatible with old WinRAR versions])
I'm also using the Inexperience Patcher (version 0.7.2, which I believe is the latest and last unless there have been unofficial updates since then [which I would be iffy about anyway), which replaces many of XP's built-in components with their counterparts from Windows 2000, as well as some elements from Whistler betas and Windows ME. On my current system (an HP ProDesk 600 G1 with a Core i5-4690 at 3.5 GHz, 8GB of RAM [XP only sees 3.4GB, which is more than enough for me], a DVD drive [not a burner, but I seldom if ever burn CDs so it's not a big deal; hooking up an external burner is no great challenge], and a 500GB HDD), XP is running with the reliability of a nuclear clock. Absolutely incredible. If you have any questions or concerns, please let us know!