Defo try a defrag of the drive. Sounds like there's a LOT to fix there. Also, how old is the drive? If constantly on, and with a large volume of files, they WILL suffer a performance hit. Try running CCleaner (freeware) and get rid of cached files, clean up your registry (make a backup of it first though), then do a system wide defrag (I like defraggler for that) and see if it helps. I would say you're looking at about 12 hours to complete those processes, but they can be left to run overnight if you want.
Otherwise, the best thing is get a cheap SSD, as suggested, or clone your drive to a separate drive of equal or greater volume (ensures data isn't lost), AND make a backup (never hurts to be safe with these things), then re-install your OS.
FYI, Windows 8 RT is a specially designed version of the OS with an different kernel architecture to Windows 8 or Windows 8 Pro. Get one of those instead. (preferably Pro of course). Honestly though, I'd stick with Win7. Win 8 is a pain in the ass and win7 is still being supported for many more years. Plus, you can run un-signed programs, game cracks etc MUCH easier than on Win 8. Not that you should be using those sorts of things...