Best program and laptop for playing against the computer?

As someone who likes to play longer games of several hours or more I often have to play against the computer. For the past few years I've been playing against Rybka 4 on a desktop with an AMD Athlon II x4 640 processor, no graphics card, and 4 gb of RAM. I have Rybka set to run at 1800 elo strength, and although I've gotten a lot more comfortable playing against 1.Nf3 and 1.c4 since it opens with those all the time, I do notice some problems with the program.

1. It seems to play weaker and stronger in different games. In some games it will often make bad obviously pointless moves like moving a rook back and forth, play  generally badly and so I can win. Other times it will play very well and I get crushed or get in a bad position and have to use all my skill not to lose. I wonder if this isn't so much the program as it might be something with my computer? Maybe the times it plays bad are when my computers RAM is tied up doing something else?

2.Sometimes it seems like Rybka 4 is actually alive, I say this because first, it sets traps and seems to be playing  with an element of hope chess. Second, there have been games where I get to a clearly winning position and it suddenly starts taking a really, really long time for its moves, so much that I begin to think  the program has frozen and then when I click to offer a draw to see if its frozen or not it immediately accepts the draw.

3. Sometimes especially during warmer weather, the program seems to tax my computer and the fans start whirring.

I was wondering for the other people who like to play against computer opponents, what's a good program to use these days and a good laptop that I can play on that won't be taxed or have its processor be damaged? I would like the program to have an adjustable ELO and be able to set it to play certain openings or have games from certain starting positions.

Any modern computer that can run Windows will be an improvement.  Without knowing your budget it's impossible to recommend something specific.  AMD usually provides a little more bang for the buck, but so many other factors to take into account.  Doubt that you need the latest and greatest but a really fast machine is kind of fun.  Probably overkill unless you want to use it for gaming or serious GM level preparation.

Stockfish and Komodo are the reigning champs of course, but if you want the computer to dumb itself down for you, HIARCS chess gets high marks for human like play.  There are hundreds of preset openings that you can practice against and it has an easy to use DB that is quite useful.  Limited to PGN files though so you have to shuffle DBs in and out and the large ones take awhile to load.  Check it out:

http://www.hiarcs.com/chess-explorer.htm

My phone can run Stockfish from <1200 to 3000 ELO, without even taxing the battery life. 

Any PC will work for an adequate computer opponent. Even ones from 10 years ago. (My PC is now 6 years old, was a budget $500 when I bought it new, and it's still overpowered for a chess computer for a nonprofessional.) 

I don't player CPU opponents often, but I was surprised with how 'decently human' the chess.com engine was at 1400-1800 levels (my guesstimate). At least in my games against it at around 1500ELO (my estimate again), most games it would set up a subtle but crucial tactic against itself that you pretty much had to find to grab a clean win. At least 3 moves deep, and not something stupid obvious or bizarre. 

Stockfish plays more 'consistently', but I've been a bit let down by its endgame play as a weaker opponent at around 1800 level - I think in its effort to 'dumb itself down', it'll play a completely ridiculously losing move in a very obvious, easy to play endgame, that even a 1200 wouldn't even consider. (Like hanging 1 of its last 3 pawns for free after fiercely defending them for the past 15 moves.) 

Thanks everyone.

I guess my budget for the laptop  is around $600 or so. I'm trying to save up as much as I can but I don't know how much longer I can work at my job which is in a movie theater as a ticket taker. It gets boring standing around there all day, even though I do chess visualization exercises in my head to help pass the time.

What's important to me is having the laptop be sturdy. My last laptop was an Acer Aspire Netbook. I had it by my bed on the floor, one morning I went into my dark room to look for something and kneeled down. I heard a crunch and realized I had put my knee right on that netbook. The screen was broke after that. So I'd like something with a really sturdy durable case, and not one that feels flimsy like a lot of the ones I've seen on display in stores.

So I did a little poking around the internet (Waaay too much free time!) and came up with a few conclusions.  You can get damn near indestructible laptops designed for military or industrial use but they're thousands of dollars so scratch that category.  Next I looked at best business laptops of 2016 according to PC magazine and they look like just what you need.  Most have some kind of magnesium/super metal case and are designed for prolonged heavy use.  Alas, they too are a bit out of your price range.  Most seemed around 12 to $1400. Only a couple snuck in under a grand.  So I have to reluctantly suggest you embrace our throwaway society and get something cheap, but new, off Ebay.  Literally dozens of choices around the $300 mark.  Yes you may have to install Windows 10 yourself and no, they won't have super rugged cases, but bang for the buck are outstanding. I was tempted to pull the trigger on a few deals myself if it weren't for my pesky credit card debt. Of course if you shop a sale at BestBuy you can get deals too.  So best of luck to you!

This one, no money, quick and easy.

http://chess.delorie.com

Sorry you had a bad experience at BestBuy, Don_frye1.  Somehow though I think there may be more to the story...

Thanks Baddogno and everyone.

As for installing Windows 10 I'd actually prefer and pay a little extra to get a new laptop with Windows 7.

If that happens, just force Rybka's move. In Fritz, all you need is to press space.

Good luck - I'm not even sure you can still get a new PC with Windows 7. Microsoft is aggressively pushing its Win 7 users to upgrade to Win 10.