DO NOT trust that OS. A common scam was to have a hacked version of Windows installed on these older laptops, and have a keylogger or something of the sort installed. The first thing you want to do is install a fresh version of windows, and NOT use any BOOT cd's that they give you. Considering that this laptop doesn't even come with the correct version of Windows that the laptop originally came with, there is a distinct possibility of this!
These run Intel HD3000. Unless you are playing games from 15 years ago, or VERY simple 2d games, gaming is basically out of the question. No, seriously, 3d is laughably bad, and even on the lowest settings at 720p you will often not hit playable FPS on even somewhat modern games. My laptop has a dedicated GPU with the k2000m, and even that will barely struggle with playable FPS on low settings with games like Diablo III and Starcraft II on 1080p. These are built to be workstations, not gaming machines. For me, I use this for programming, homework, movies, music, and old school roms/emulators on the go, so for me, it is perfect. Again, GAMING = NO.
Now that I am done with the disclamer on some big issues, onto other issues! YAY, isn't this fun!? Also keep in mind the t420s is slightly different than the t420.
- The t420s has a soldered CPU, so it is NOT upgradeable
- t420s has a USB 3.0 Port, while the t420 does not. It also lacks a couple ports the t420 has, though I can't remember which
- the s can NOT take the larger extended batteries the t420 can, though it can take a three-cell battery in the expansion bay for a total of 6 cells. Note that these tend to die very quickly and often need replacement more often than regular batteries due to power management on the laptop.
- The regular t420 can be upgraded to a 9 cell for better battery life
- The s uses a thinner standard for its optical drive/HDD/SSD expansion drives, meaning your upgrades may be limited, and not all normal 2.5" drives will pop right in.
- the t420s tends to thermo-throttle
- Most people regret getting the t420s, and prefer either the t420 for a better overall laptop, or a x220 for a smaller laptop with a better screen
Now with all that said, I was in the exact position you are in now, and I myself ended up getting a w520 with a quadcore i7-2720QM, upgraded to 16GB of ram, a 1TB SSD, and a 256GB SSD. I freakin love this thing to death. The keyboard is everything people said it was and then some, the red nipple is so much better than a trackpad that I never want to go back, the screen is WAAAAY better than the t420/430 series its almost laughable (I got the 1080p version), it has a MUCH faster CPU and the cooling to handle it that I never have had any performance issues on this laptop (several people who upgraded to a quad core i7 on the t420 had issues with thermos), and I can have up to 32GB of DDR3 1866 ram, 3 hard drives (though 2 run at SATA 2 speeds), an ethernet port, Displayport, VGA, ect. Considering it is the only computer I have at the moment, I couldn't be happier, other than possibly moving to the t460p that just got announced, not so much for a speed upgrade, rather for a size downgrade while maintaining the current speed I have...ok maybe the integrated graphics will improve, but as mentioned before, I don't use anything that would benefit from that; this w520 is not a small thing, but damn do I love it.
My recommendation if I were in your shoes would be an x220 with IPS display (MAKE SURE it says IPS), or if you absolutely need some processing power, a t420. I would pass on the t420s. Also, if you have the budget, a t520 and w520 are also great machines too.