Reload that machine with Windows 7 64-bit home premium or professional, and use either Chrome or Firefox.
Without even changing the hardware, you'll experience computing like never before.
Though it probably would be worth considering a new build. Something in the Haswell family from Intel with 8-16GB of ram and 64-bit Windows 7. Windows 8 is not recommended for desktops unless you are building for a kid who is used to smarphones, and Windows (9) 10 is probably going to be a disaster.
Telling someone to reload their machine with a different operating system doesn't seem like the desired solution to a particular software problem that's just emergent.
It is indeed far from ideal- but in this case there's a good chance he's facing a compatibility problem.
I've got a number of Windows XP machines still in use in an industrial setting, and I've noticed that recently a growing number of sites have dropped compatibility for that version of internet explorer. Upgrading to Windows 7 should let you regain the performance of a newly installed OS, while at the same time bringing compatibility up to where the software he wants to use will probably run right again.
Windows 8 is internally similar to Windows 7, but where the past few Windows versions have required a 'housebreaking' period of sorts where you get everything arranged the way you want and get used to its interfaces, Windows 8 has the the most involved learning curve because the UI is so different. It is possible to install third party addons that bring back the layout people are used to (circa 1995 after all), but this still is time consuming and results in a machine that has been customized and could become harder to troubleshoot later on.
With any luck Windows 10 will go back to having the traditional layout as the default, but I am not counting on it. Microsoft has delivered disappointments year after year, and the only reason I still install their stuff on any machine is compatibility with what people what to do. Every few years I try using Linux instead, and its always the compatibility headache that sends me back to Windows.