Description of your new PC sounds solid and yes you can use storage drives that you already have in your new PC. You'll have to install Windows one more time, but you don't have to lose your data. If I were you I'd buy your mate a beer or two and ask him for help with that.
Going back to storage drives. As I said before you can have a lot of those in your system and you have two types of drives currently available on the market:
1) HDD (hard disk drive). Old time-tested technology (kinda amazing thou). The main pro of it is that you can get a lot of capacity (the amount of GB) for relatively low price. For example, you can get THIS popular 1TB HDD for the low low price of 42 quid. Main cons of HDD drives: they run hotter (thou that's almost never an issue), they are kinda fragile (there's a lot of very intricate, sophisticated "stuff" inside) but unless you drop it on the floor this also won't be a problem. The last, most important con is that they are kinda slow. In the last decade or so processors and graphics cards got kinda fast and HDD's stayed the same therefore HDD is usually the one element lagging behind all the others if you need your computer to do something fast.
2) SDD (solid state drive). New-ish tech (they are way more mainstream nowadays than they were like 2 years ago). Main pros: they are really fast, they require almost no power and therefore they stay cool during operation. Also, they are much less fragile, you can drop one on the floor and it will probably stay alive and well. Main con: price. You get much lower capacity for a higher price then if you bought HDD (EXAMPLE), but the speed is amazing. Once you use PC with SSD in it you won't go back. Ever. SSD's are also important for poker because Holdem Manager and Poker Tracker operate on very large amounts of very small files and HDD's don't like that.
If you have both SSD and HDD in your system you can make the SSD into your main"C:" drive (if you don't know how ask a mate for help or if he can't do that for some reason get back to be and I'll help) and keep everything you want to run fast on it (Windows, web browser, poker room, Holdem Manager and other important software) and you can make your HDD into "D:" drive (or split it even further into "E:", "F:" drives etc.) and keep all of your data there (movies, music, games etc.).
Hope that helped you understand the issue. If you have any more questions let me know.