Well, you're in luck!
www.gog.com is currently having a sale on classic DnD rpgs from the 90's, and they are pretty much all fantastic! And every single game on this list is either free or 3,49 US dollars(except for the steam game)
If you want more directed advise, then I suggest
Planescape: Torment, which is often considered the greatest video game story of all time, and
Baldur's Gate 1+2, which are considered the second best story of all time. Even though the first is just needed for context for the second one, where things really pick up.
However, I do not know how familiar you are with the Forgotten Realms universe and rule set. I wasn't and it was pretty hard for me to get into them, but well worth it. So, if you want something a bit easier to get into I'd suggest
Fallout 1. It has a great story, teaches you everything you need to know about the mechanics from the manual and has a very interesting post-apocalyptic world. I really like the combat as well. I assume that you've heard of the series at some point in your gaming career.
On a similar note there's also
Arcanum: Of Steamworks and Magick Obscura. Steampunk fantasy. I have not personally played this, but it is held in very high regards for basically the same reasons as Fallout. Well fleshed out, very interesting world, great combat and self-contained mechanics. Comes highly recommended (mainly cause I wanna know what you think)
If you want something not as tactical and not party-based, and a bit more modern as well, then pick up
Vampire: The Masquerade: Bloodlines. Modern day, secret society, vampire clan wars, with some werewolf and flesh manipulation thrown for good measure. Again: praised story, great setting etc etc. Sorry i'm just starting to get really tired of writing

. The gameplay is basically Deus Ex, although not as well executed, mainly due to budget constraints. Don't let that discourage you though, this is up there with the all-time greats. (Just make sure to install the unofficial fan patch, the game's unplayable otherwise, and you can find it in the game's steam forum).
Just to cover all my bases and you just happened to be talking about MMO's, then I recommend
Elsword Online a F2P Beat Em' Up rpg which basically does everything right. It's very easy to get into, and is very easy to just have casual fun in, as well as hardcore end-gamey stuff in. Cash isn't needed either, since the cash shop isn't obtrusive at all. watch this video if you wanna know more:
http://www.youtube.com/watch?v=ncEzChIE-OY
It's from a regular dude, who isn't an MMO expert, but does a very good job to explain a MMO's good and bad points, while giving an overall look at the game.
If you want something a little more similar to WoW, then I recommend
Eden Eternal. I'm really getting tired now, so I'll just link to the same dude's video about that instead.
http://www.youtube.com/watch?v=bOrmRbE07cs
You can find all these games on
gog.com except Vampire: The Masquerade: Bloodlines which you can find on Steam (and the mmo's which you can find on their respective sites by googling). Some of them can be found on Steam as well, but Steam doesn't ensure that they are compatible with your computer, while gog does. Anyway, hope I have been of some use.
And that concludes the longest post I have written on here.
EDIT: By the way, I have assumed your PC is a midrange pice of hardware from 2007, so you should no problem running any of these. Also, I have no idea why the gog links don't work, but you can google it.